If you wanted to know when they stopped being a pack, you would need to consider the opinions of all of the members of the (now former) Hale Pack:
If you asked Scott, he would look at you with the steely expression of someone freshly reminded of great pain and say, "How should I know when Derek's pack fell apart? He's not my Alpha, and we're not Pack." Allison (rarely very far from Scott) would look at him with sad eyes, but would not say anything.
Isaac would tell you about how Derek distanced himself even further from the pack than before until, by the time he was halfway through his first week of senior year, there was no way to fix what was wrong.
Asking Jackson would result in a glare and a roll of the eyes, and, before he can answer, Lydia would speak up, demanding he drive her to the store while giving you a look that would be utterly terrifying... if it weren't for the heartbreak only just hidden in her eyes.
Danny, blinking at you like you had asked a stupid question, would shrug and carry on with what he was doing before you stopped him with a barely audible sigh.
Derek would be more difficult to communicate with, but if you could read his mind, he would be thinking of a date – the first Thursday of the younger Pack members' summer vacation after junior year.
Peter would smile and tell you, quite openly, that the pack dissolved and everything went to hell on Christmas Eve of the Pack's junior year.

You wouldn't be able to ask Stiles.

Not for lack of trying, no. But Stiles no longer attended Beacon Hills High School. He received an early acceptance for an online course with Oxford University, and was going to be studying mythology and ancient languages. As soon as he got the letter, he ran out and bought two brand new laptops, suitcases and began packing. He sat down to dinner with his father the next night and discussed him travelling while doing his studies. After arguing over what would be best for Stiles and about whether or not Melissa McCall should come over every once in a while to check on the sheriff, Stiles taught his dad how to use Skype so that he can keep in contact easily (Stiles wanted to travel the world, so taking his cell phone would end up costing too much if he stuck with his current number, and would get too complicated if he changed his number every time he moved country).
He didn't tell him that he just didn't want to be called by a certain pack of wolves every time they needed him to do research on some big bad beastie that had just kicked their butts while he was trying to sleep anymore.

Stiles finished his final week of school, and tried several times to tell someone, anyone, that he wouldn't be coming back for senior year. That he was leaving the next weekend, and wasn't planning on being back for at least a year.
Scott was too wrapped up in Allison to pay attention to anything he said.
Stiles still couldn't look Allison in the eye after what he had gone through at the hands of her family (he knew she had nothing to do with it, but still).
Isaac was still Derek's lackey, so would still snarl at him if he got too close.
Jackson was still a tool, and he and Lydia would never have bothered listening to anything Stiles had to say.
Danny would deflect every time Stiles tried to speak to him, assuming Stiles was asking him if gay men would find him attractive, or any other question that he had asked since Danny came out.
Peter... Peter was Peter, an un-dead creeper-wolf that had tried to kill him in the past and still bad-touched him – he refuses to go anywhere within twenty feet of him.

Stiles walked up to the Hale house the Thursday before he was set to leave, having seen Peter stuck at the supermarket. He pushed open the old charred door, and listened to the silence in the house, realising that the pack must be, once again, training without Stiles there. Seriously, how was he supposed to not be the pathetic, weak human if they never let him train and get stronger with them? He sighed and wandered into the dilapidated house, determined to wait for people to show up so that he could finally tell someone – could finally brag about being out of school for good, could discuss where they thought he should travel to while he was away... see if anyone tried to talk him out of going.
Walking around the sofa, he only just managed to stop himself from just throwing himself onto it as he realised that Derek was sleeping there, all signs of his stress and anger gone from his face and an old hardcover book lying open on his chest like he had dozed off while reading. Stiles suddenly found himself kneeling beside his sourwolf (although he would never admit how he thought of Derek to anyone) his cinnamon eyes dragging over the sleeping form, memorising how he looked when he was relaxed and his features had softened. Stiles froze when Derek stirred, blinking slowly. He looked at Stiles through bleary eyes, gave a small, almost silent, laugh and murmured, "I've missed this dream," before reaching out, pulling Stiles to him and pressing their lips together gently.
Stiles remained still, completely shocked, for a few seconds as Derek kissed him far more delicately than he would have that the wolf was capable of. He blinked dumbly as Derek pulled away from him and blinked sleepily before burying his face in the crook of Stiles' neck, inhaling deeply. Stiles moved his hand slowly to rest on Derek's head, stroking his hair gently and tangling his fingers in the thick locks. Derek nuzzled Stiles' neck before mumbling into the human. Stiles started to ask Derek to speak up for his pathetic non-super-wolf hearing, but trailed off when he heard the quiet snores the Alpha was releasing onto his collarbone. He slowly lowered Derek back onto the sofa before sighing and hugging himself, not wanting to leave without being able to say goodbye to anyone. His gaze darted around the room one last time as he mentally prepared himself to walk out of the Hale Manor – and the Pack members' lives – forever, when he heard a quiet, pathetic whine of "Stiles" coming from the direction of the sofa and felt his heart break into shards that cut through his chest.

He would never be prepared to say goodbye to them.

Stiles and his father had a deal – Stiles wouldn't call home unless he really and truly needed to, and the sheriff would wait until Stiles called first to call him. So when John Stilinski dropped his son off at the airport, it was understandable that he didn't want to let his seventeen-year-old, hyperactive son go. He helped Stiles as far as he could through the airport, and just before they had to go their separate ways he pulled his son to the side and handed him the carrier bag he had been fiddling with the whole day, "Here, it... it was your mother's. She wanted me to give it to you for your eighteenth, but I'm sure she'd be okay with me giving it to you now." The sheriff shook his head slightly, voice thickening with grief, "She would have been so proud of you, she... God, Stiles–" he threw his arms around his son and held him close for one last time before he went beyond where he could reach. Stiles hugged him back as tightly as he could, tears springing to his eyes as he clutched his father the same way he had on Christmas Day when he was eight.

Stiles held his mother's scrapbook gently in his lap, eyes flicking between the words and images she had painstakingly annotated and sketched, feeling closer to her than he had in years. He skipped a few pages, and stared at a familiar pair of eyes staring back at him from a photograph. His mother stood in front of the Hale Manor, her arms outstretched and a huge grin on her face, her left hand grasping at that of a petite blonde woman with red lips stretched into a mirth-filled smirk, with eyes the same hazel that he had seen so many times when pushed up against walls. The woman lent against a man who looked very much like their son did now, his black hair out of his face and his stubble-covered cheeks taut as he glared at the cinnamon-eyed angel holding his wife's hand. Stiles ran his fingers over the picture, smiling softly at little Derek hiding behind Claudia Stilinksi's skirt while Laura and two other small children, all bigger and probably older than Derek, ran around in the background. If Stiles looked carefully, he could just about make out the slight bump of his mother's pregnant stomach. Peter Hale stood slightly to the left, arms wrapped around a woman with plain features in comparison to the two standing at the front of the group, but heavy with child (first child if the proud expression on Peter's face was anything to go by) and Stiles was suddenly struck by just how much Peter had lost in the fire, remembering reading somewhere that wolves mated for life, and the loss of their mate was next to unbearable even for the most resilient of wolves.
But what really grabbed his attention was the caption written twice in an unfamiliar hand – once in English, then again in Russian: My Pack and my fayn. Stiles made a mental note to look up what a fayn was when he landed in London.

Stiles checked his phone while he waited for the guy behind the counter to weigh the scrapbook so he could send it to the address he was staying in while in London. The flight was pretty booked, so he wouldn't be able to keep it on the seat next to him, and it was too big to fit into his carry-on luggage. He signed the papers thrust in front of him and pays the fee to get it there, before going to sit by the gate and wait the half-hour left before he can board his flight. As he sits in one of the uncomfortable, sort-of-but-not-quite-actually-not really-at-all-plush chairs he looks up to see a girl wander over looking lost, her gaze darting between the ticket clutched in her hands, the sign for the gates and the few seats remaining at gate 19. She gnaws on her bottom lip briefly before steeling herself and approaching a seat a few people over from Stiles, her white hair and pale skin holding Stiles' attention as she folded herself onto the chair, red eyes constantly flickering up as if to check for anyone coming her way to attack her.

When the air hostess called for all passengers to begin boarding, the girl shot up and was one of the first to board, dodging other passengers in her hurry. She kept her shoulders hunched and her head down, avoiding all forms of contact and flinching any time someone brushed up against her. Stiles couldn't help but watch her, curiosity getting the better of him. As he approached the counter to hand over his ticket, his phone began to howl as he got a call from Scott, causing people to turn and stare at him as he struggled to get it out of his pocket, his own eyes snapping to the girl as she jumped and flailed, looking around in panic. "Scotty-Boy! How are you this fine morning? Is there anything in particular I can help you with within the next five minutes, or were you and Danny just finding out whether or not he actually could turn on someone's cell phone remotely? Because if you were, I'm glad you did it now and not some time during the next eleven or twelve hours because I really don't want to–"
"Stiles." Ah. That's not Scott. "Where are you? And why are not at the Pack meeting?"
Stiles sighed, "Y'see, this – this right here – is why we need to get our heads out of our asses and listen to Stiles every once in a while, because he might be trying to tell you something important like, oh I don't know, maybe... Who else is there?"
What? Why do you need to–" Stiles rolled his eyes at the annoyed confusion in Derek's voice, and was almost certain that the Alpha sourwolf would be able to hear the smirk on Stiles' face when he continued his rant.
"I want to know how many of the other non-listening grouches are going to call me later and potentially get me kicked off my flight. Forget it, Grouchy-Face, just put me on speaker so I can speak to everyone there, and you can pass the message on to anyone who isn't." Stiles waited, listening to Derek's growls on the other end of the line, before hearing a tinny affirmative in Isaac's voice to carry on, "So... I'm on my way to London."
Stiles was expecting the shouts and the anger – he was technically only just telling them now, after all, intentions didn't count in this situation – but what he wasn't expecting was the "What" uttered in the most dejected and meek voice he had ever heard, and wasn't that just a stab to the heart just there?
"Look, I've been trying to tell you guys for – literally – months that I got onto the Oxford course, and for almost as long that I've been trying to tell you that I'm moving – it's not my fault that not a single fucking one of you assholes bothered to listen to me."
To Stiles' surprise, it was Jackson that spoke up next, "Look, Stilinski, just wait for a minute, okay? We'll... fuck, we'll come to the airport and pick you up, bring you back–"
"Are you actually shitting me right now?!" Stiles couldn't believe this, he was finally getting out of Beacon Hills, getting a chance to travel and see the world, and all his 'friends' could think of doing was bringing him back and keeping him there, "If I don't go, I lose my scholarship. More than that, I lose my chance to go to Oxford at all. I'm not missing out on this once-in-a-lifetime opportunity because all of you are sulking because you refused to listen to me when I tried to say goodbye. I'm sorry, but this is important to me, and I can only give up so much of my sanity, my time, and my life before I give up everything entirely and put my dad's gun to my head." Slight exaggeration, but Stiles needed to get his point across.
"Fine!" Lydia's voice rang through his soul, "Fine, just wait at the door so that we can at least try to say goodbye to you properly, please."
"You've had your chances; you've had plenty of them. Besides, I kind of can't do that now," Stiles handed his passport and boarding pass to the woman behind the desk and held his hand over the microphone while she briefly told him that he would need to be off his phone when he was on the plane, knowing full well that the wolves would still be able to hear her every word clearly, "because that was me going through the gate, and this is now me boarding my flight which leaves in twenty minutes and even if you all piled into the fastest car you have and drove it the fastest it can go, you'll still only get to the parking lot five minutes after it takes off. This... this is the best goodbye I'm gonna get from you guys. So when I come back I'm expecting incredible things. I'll see you then," Stiles took a breath which sounded more like the desperate gasp of a drowning man, "YA lyublu vas vsekh, vy bespolezny durakov*."

He just managed to catch Derek's gasp before he hung up.

Stiles double checked the seat allocation on the boarding pass and above the seats before hoisting his backpack into the overhead compartment and plopping himself down into his seat, accidentally-on-purpose startling the girl from earlier. He watched her out of the corner of his eye – watched her nose wrinkle as she inhales, the way her eyes widened and she went into a contained panic after a second, the way she turned and looked at him, the way her features relaxed then creased as she took in the sight that was Stiles Stilinski.

After about twenty minutes of awkwardly avoiding-eye-contact-but-still-watching-each-other , Stiles metaphorically threw his arms in the air and thrust his hand towards the girl, turning his body so that he was facing her fully, "Look, my name's Stiles – Stiles Stilinski. I'm on my way to meet my professors before taking online courses at Oxford. And, while under any other circumstances I wouldn't really be bothered by a girl as pretty as you – or anyone else, if we're going to be honest – staring at me as much as you've been, if anything I would be, and still kind of am, flattered at even the thought, but right now we need to get something straightened out," Stiles lent in further towards the girl, shifting himself as far as he could from the passenger in the aisle seat, although he seemed to be so engrossed in his movie that Stiles was pretty sure he wouldn't notice anything they said at all. He lowered his voice as much as he could, "I'm not a threat to you. I don't care what I smell like to you, I'm human. Yeah, I hang out with a pack of wolves in my free time, but I'm not a werewolf. Okay? I'm not going to hurt you."
Her scarlet eyes flicked up to his cinnamon ones, watching for his reaction before she let out a breath and smiled, fangs showing slightly, and a blue glow flashed over her eyes, "My name's Nikola. Nikola Richards." She took Stiles' hand in her own, "Most people just call me Nikki, though."

Stiles and Nikki spent their time getting to know one another; he found out that she was an Omega wolf looking for a pack to join, had grown up in a small town in England and had the exact same sense of humour as Stiles did. Stiles told her about his life since Scott turned into a werewolf, complete with psycho-now-undead-and-bad-touchy werewolf (although Stiles felt a twinge of guilt as he gave that explanation) and Grumpy-McGrowly-snarly-pants sourwolf.
That's when the plane jerked.

The entire plane jolted, shook, then the pilot's voice came over the intercom, calling for all passengers to return to their seats and prepare for an emergency landing.

As soon as Stiles hung up on them, Lydia grabbed her phone and looked up what flight Stiles would be taking based on what he had told them. Finding five flights that fit the description, she checked with Danny where Stiles' signal had come from, and asked the wolves if they heard what gate Stiles was boarding from. Filling those details in, she found the flight Stiles was taking and began tracking the flight. Derek came back in after half an hour outside, twigs and leaves stuck to him, cuts healing and anger boiling behind his eyes. He snarled for a training session, and growled for Allison to begin training Danny and Lydia to fight and defend themselves from witches, demons, rogue wolves and the likes.
So that's what they did. For the next four hours they trained mercilessly in the forest, Derek not giving anyone a chance to back down or take it easy, pushing everyone to their limits over and over again. Eventually Lydia's phone started going off repeatedly, the sound she had set for any tracking updates for Stiles' flight. She looked over to Derek, silently asking his permission to check in. He didn't look at her, just growled his consent.
Lydia sprinted to her phone, hands shaking as she picked it up and unlocked it. It's too early for them to have landed yet. What... Lydia froze as she read and reread the message. Her hand flew up to her mouth and she screamed – an awful, heartbroken shriek crawling up from the pit of her stomach that caused the wolves to come running to her. Jackson took the phone from her hands as she swayed, her legs starting to give way, her face deathly pale and tears already pouring down her face and she looked up to the Pack. Her mouth opened and closed soundlessly and she just shook her head and buried her face in her hands. Allison gently pried to device from Jackson's fingers so he could attempt to comfort the strawberry-blonde and her eyes widened as she read through the post. She cleared her throat and began to read it out loud:

"Flight AA7783 from LAX (California, USA) to LHR (Heathrow, UK) has just crash-landed into the North Atlantic Ocean. Emergency services are on their way to locate and assist survivors. Reason for attempted emergency landing remains unknown. Number of survivors is currently unknown. More information to follow as it happens."

Twelve.

There were twelve survivors in total. No names were given, but Lydia automatically did the math and understood that the probability of Stiles being one of those twelve was ridiculously low, especially considering how recently he had taken to being almost Derek-like in his levels of self-sacrifice. Needless to say, none of the Pack slept that night, or the next night, or the next. All of them were waiting desperately for a phone call from their hyperactive friend, or a message in the newspaper about the tragic death of the sheriff's only child.
They waited for a month, before assuming the worst and giving up what little hope they had that Stiles was still alive.

After another month, Derek received a text from an unknown, UK based, number. "I'm sorry. I miss you all, and I love you." He didn't feel the need to tell the Pack – didn't want to get their hopes up if he wrong. He really wanted to be right about this.

*I love you all, you useless fools.