Thanks everyone who has followed/favourited this story so far!

Special thanks to HT188, xXRosexScorpiusXx, iwantsprezzatur, creelluka and FromEyesToMind for their lovely reviews.

Actually some of you have told me that it's odd that the Slytherins were surprised at Emma taking Muggle Studies. What I meant to write that they were surprised at her taking that class at N.E.W.T. level (since some don't like Arithmancy, Care for Magical Creatures or Divination, Muggle Studies was one of the only options). Plus, they're all a lot more anti-Muggle than they were in third year. Hope that clears stuff up!

P.S. Creelluka - Snape as Slughorn should be someone's Boggart :D You had me laughing.

A/N:

James got turned into Emma and vice-versa.

Regulus got turned into Sirius and vice-versa.

Rabastan (much to his disgust) swapped places with Hestia the Hufflepuff Muggleborn

Lily was protected by Slughorn's "bubble spell".

Marlene McKinnon and the rest of the class got a dose of Polyjuice Potion, but who cares who became who?

'He's off his rocker!' Rabastan exclaimed in a girly voice. 'Using the Tongue-tying curse so that we can't say who we really are? Did you do that on purpose Potter?' he rounded on James-as-Emma.

'Relax Lestrange,' it was odd for Emma to hear her own voice. 'Trust me, if it was up to me, I wouldn't be the target. Anyway, we're stuck like this, so you'd better get used to it. Luckily for you, you have a free period.'

The three Slytherins made their way down to their common room, Rabastan complaining about being a filthy Mudblood all the way. However just as Emma was about to say the password, Mulciber and Snape appeared.

'Well, well, well,' Snape said. 'If it isn't Potter and Black. Lost are you? And what's this? Oh, a Mudblood present for us to play with. You shouldn't have.'

'You got no business being down here,' Mulciber said, waving his wand around thoughtlessly. Emma kept an eye on it.

'You don't understand, we're…Ah!' Rabastan choked as his throat locked. He glared at Snape, who was intelligent enough to realise what had happened. 'I hate you.'

'Maybe you should have been nicer when you spilt my pumpkin juice this morning,' Snape said snarkily. Mulciber looked confused. 'Now get out, before I actually do hex you.'

'Well that was idiotic,' said Regulus, annoyed. He angrily flipped his hair out of his eyes. 'And why can't my bloody brother cut his hair properly?'

'Hey Sirius,' a Gryffindor girl with big brown eyes batted her eyelashes at him. Emma held in a laugh.

'Uh…Hi,' Regulus nervously ran his hand through his hair, not knowing what to do.

She smiled at him expectantly, before noticing Rabastan uncomfortably tugging at his Hufflepuff tie, his shirt buttons undone.

'How could you?' she rounded on him angrily. 'I mean I had heard about your reputation, but did you really just forget about me in one night? We've been in the same House for years. I can't believe you've already found some slut.'

Rabastan didn't even try to protest. He wasn't about to defend a Mudblood's honour and he was still uncomfortably wearing his old clothes beneath his robes. Emma dug around in her back and offered him her Quidditch jumper.

'It's not like that...' Regulus trailed off weakly, shooting a look at his friends. Oops, Emma thought, realising how bad their actions had looked.

'Oh really? Then what's my name Sirius?' the girl demanded.

'Umm.…' Regulus cast his eyes around helplessly. Rabastan and Emma looked at him with pity. 'Katie?'

'Not even close,' she replied, slapping him and storming off.

'You know what Prongs? I think I could get used to this,' they heard Sirius using Regulus's voice from behind them.

They turned around to face themselves. Emma blinked several times in surprise. Rabastan was busy swapping his jumpers.

'Nice look Lestrange, but make sure to wash that jumper before you return it to my sister,' James said, smirking. 'She likes her clothes clean.'

'As if I could even wash out the stain of a Mudblood,' Rabastan retorted, before Emma could reprimand her brother. She rolled her eyes. Honestly, those two are more alike than they realise. Rabastan doesn't even mind Gryffindors...

'Hey Ems, remember to scruff that hair up a bit,' James mussed up her - or his - hair. 'Wouldn't want Evans walking by and not recognising me.'

'Did someone mention my name?'

'Salazar, is the whole Potions class in this courtyard?' Regulus mentioned darkly.

'Nice to see you too, Black,' Lily replied coolly, before grinning smugly. 'Actually I was just about to tell you that we have Defence Against the Dark Arts. We wouldn't want James and Sirius to be cutting class. Besides, I'm curious to see what would happen today: I can't wait until lunch.'

'Lunch at the Hufflepuff table,' Rabastan groaned. 'I think that this witch is going to call in sick for the day.'

'Where would you go?' Emma asked matter-of-factly. 'It's not as if you could go back to the common room, and besides if a Slytherin catches you alone...' She stopped herself, remembering that her brother was still there.

'Oh don't stop that sentence on my account,' Sirius said snidely. 'I know sweet Reggie has sensitive ears, but we know what you Slytherins do for "fun".'

'Says the person who hexes people at random,' Emma started, before Regulus grabbed her arm and dragged her off to DADA.

'C'mon,' he said. 'I can't stand that git.'

In the DADA class there were a lot more students. It was for Gryffindors and Ravenclaws - Hufflepuffs and Slytherins having the class on a different day. Emma hurried into a seat, not realising that it was next to Lily Evans. I guess this is my chance to get to know who made my brother fall for head over heels. The teacher called for silence.

'Today we will be starting on the famously difficult Patronus Charm. Now, Professor Flitwick and I will give complementary lessons on this, so to succeed in one is to succeed in the other. The objective of this Charm is to repel Dementors, one of the foulest beings on this Earth. There have been rumours of the self-proclaimed Lord Voldemort amassing an army of them, so we felt it was in your best interests that you learnt as quickly as possible. However, as you all know, a fully corporeal Patronus is immensely difficult, even for wizards having graduated Hogwarts, so a non corporeal one will do.'

Most of the rest of the class was spent studying the theory of the Charm, but fifteen minutes before the end she had them practicing.

'Think of a happy memory. The happiest you can. Hold it firmly in your mind. Do you all have one? Good, now repeat after me: Expecto Patronum!"

A silvery rabbit sprouted out of her wand to hop around the room before disappearing. Soon the whole classroom was filled with incantations that progressively became more and more annoyed. In the end, Lily Evans was the only one who had issued a white smoky substance out of her wand for a couple of seconds. The teacher awarded 10 points to Gryffindor.

'Well done,' Emma congratulated her, though refrained from asking her what her happy memory was. 'That was really hard.'

Lily looked at her for a second in surprise: she obviously wasn't used to James talking to her like a normal person.

'Thanks Potter, I mean Emma,' she replied. 'Can I call you Emma?'

'If it's the only way you can differentiate James and I,' Emma said with a laugh. She glanced back: "Sirius" had been approached by another fan girl. Lily saw the look.

'I'm actually surprised there aren't more girls after James,' she admitted. 'I mean, even I can concede that he's good-looking, even if he is an arrogant toe-rag. Sorry.'

'That's okay,' Emma replied. 'It's true. But James really likes you, Evans. Try not to crush his heart too much.'

Lily laughed. 'Call me Lily. We've been on enough patrols together to be on first name terms. Anyway, I think it's become a habit to ask me out, honestly.'

'No,' Emma said. 'It's not.'

And she left Lily to ponder her words.

In Charms class, James was struggling. So he was bright, that didn't mean he studied every single subject over the summer. Besides, he was too busy missing Evans. Her infectious laughter, her brilliant green eyes…It may have started off as a joke, but now he knew she was the person he wanted to marry. He just had to make her realise that too. He glanced over at Sirius, who was examining his tie in disdain. Poor Padfoot, he's been trying to go against his family's wishes for years, and today he's in some kind of parallel universe: what might have happened if he wasn't so brave.

An unfortunate pug-faced girl sidled up to "Regulus".

'Is there something wrong with your tie?' she asked. 'I can fix it for you if you'd like.'

To James's surprise, Sirius allowed her to. Then again, it was Sirius. The girl shot a worried look in James's direction, as though she was scared that he would tell her off. James thought it would be funny to wink at her. She blushed and dropped Sirius's tie as though she had been burned. Interesting, James thought. Obviously Emma knows this girl fancies Regulus. I wonder if she's her friend? If so, he was disappointed in his sister.

He had no idea how this girl had made it past OWL levels. They were doing a quick revision of all the fifth-year level spells, and so far he hadn't seen her master a single one. A slip of paper slid itself onto James's desk. He unfolded it: What do you think? Should I get Reggie his first girlfriend?

James grinned and sent it back: I'm all for it, but I don't think Emma would approve of their getting together…Good thing I'm Emma today.

"Regulus" turned around with a wide Sirius-grin that was unnerving on his taciturn little brother.

'Please don't ever do that again,' James said, shuddering.

He wasn't sure he liked Regulus. He was too… cool, too distant. Though his sister seemed to have taken a shine to him. Then again, could James blame him, with the parents he had? But this girl - Sophie, he found out when Professor Flitwick reprimanded them - obviously liked this emotional Regulus. She probably thought that she was the one to get past his "mysterious" attitude. He snorted and went back to trying to silence his toad. It croaked pathetically, but it was still making noise. Next to him, Sirius had turned on a different type of charm.

'You know I could help you with that,' he nodded towards the pile of paper that Sophie was supposed to burn.

'You're so kind, but I think Flitwick would notice,' she tried to use the spell again, but only succeeded in enlarging the paper.

'I meant I could tutor you,' Sirius clarified, and then lowered his voice, staring into Sophie's eyes. 'Privately.'

James thanked Merlin that Lily Evans seemed immune to Sirius's charms. She was probably the only one. Even Emma seemed to have succumbed to a snog at the Yule Ball, though James had made sure that Sirius would never attempt that again. Apparently the threat of having his hair shorn off was greater than the attraction of his twin. Good.

The bell rang to mark the end of the lesson, but Flitwick called him back. 'Miss Potter, a word?'

'Yes Professor?' James dragged his feet to the front desk, wondering what kind of trouble he had gotten his sister into on the first day of lessons.

'Are you quite alright? I noticed that you had some trouble with your spells today,' the wizened old teacher looked worried. 'I hope you aren't overworking yourself.'

'Why would I be overworking myself?' James asked, nonplussed.

'Quidditch captain, Prefect,' he pointed to the badges on Emma's robes. 'I also know that you've been working hard and aspire to be a Curse-breaker. I would advise you to relax for a couple of weeks. You're exempt from the homework. It's the first day of school, you can afford to have some fun whilst you can.'

With that he bade James farewell, a little twinkle in his eye. So, Flitwick seems to have a soft spot for my sister, James thought with amusement. Lucky beggar, Flitwick assigned us a ton of homework. He then realised that he hadn't had Charms class yet. Maybe we'll get less, he thought hopefully. He didn't know Emma wanted to become a Curse-breaker though. He realised that there was a lot he didn't know about his twin anymore.

The rest of the day passed awkwardly for Regulus, who had learnt to perfect his imitation of Sirius quite well, but still had no idea who all of the girls were, resulting in a couple of jinxes and lots of hurt looks. Emma teased him for being a heart-breaker, but he was not amused. At lunch, Remus commented that Sirius seemed out of sorts, which he attributed to their stolen fake wands in Transfiguration, and that James was quiet, which he thought was a new way to grab Lily's attention.

For their part, Sirius and James exchanged horrified looks at the casual way the Slytherins mocked Mudbloods. Twice, Sirius had tried to defend Muggleborns and James elbowed him. He may not like it, but if they wanted their siblings to still be alive at the end of the year, they had to pretend to go along with it. James found the perfect opportunity to fit in when Lestrange walked past as Hestia, and they had great fun hexing and jinxing him until he was sent to the hospital wing with six tentacles sprouting out of his purple face, frogs leaping out of his mouth whenever he tried to say something. James realised that maybe they had taken it too far when they saw Mulciber's overjoyed expression.

'Just like in fifth year!' he said enthusiastically, shaking James's hand until he thought his arm would fall off. Fifth year? I really don't know anything about Emma's life.

'These people are sick,' Sirius muttered to him as they went to find Hestia and see how she fared.

It turned out that she found them: they saw Lestrange run past them with strawberry blond hair.

'Finally!' Sirius fist-pumped the air. 'Can't wait to be rid of my smarmy little brother. I can't believe half of these kids are scared of him, I mean have they seen what a wimp Regulus can be? Don't they know he's scared of drowning?'

James shook his head, but fervently agreed. Being his twin was weird, especially because she was a girl. He kept on accidentally touching his breasts when he crossed his arms and freaking himself out. Of course Sirius found it immensely funny. Plus, James didn't know how long he could take the older Slytherins checking Emma out without hexing them. And to make things worse: no Lily.

But even after dinner they still looked like their siblings. James looked over at the Gryffindor table and caught his twin's eye. She widened them, and pointed to the ceiling. It was already dark outside, after eight o'clock at least. Their Potions lesson had finished at 10.

'Excuse me,' he said, walking out of the Great Hall. A couple of minutes later, he was looking at himself. It was creepy.

'What happened?' Emma demanded with his voice. 'Why isn't the Potion wearing off? All of the others have, McKinnon came back to the Gryffindor table two hours ago. I heard her and Lily talking about it.'

'I don't know...' James said. 'We need to go and see Slughorn.'

'I'm sorry,' Slughorn shook his head at the four of them. 'I forgot that the Polyjuice Potion has an odd reaction to family members. Your DNA is too similar for the returning effect to be triggered.'

'You mean we're stuck like this?' Sirius squeaked.

'Please do not make that sound out of my mouth again,' Regulus narrowed Sirius's eyes at his brother, who rolled his eyes.

'Okay this is not at all freaky,' Emma said. James had to agree: with the four of them together it looked like some sort of pantomime for people who knew them well.

'Please, children, don't panic. I've heard of this before. All it requires is a simple Memory Potion mixed with Dittany. However, I won't be able to procure the antidote before tomorrow.'

He closed the office door and the four of them were left staring at the panel of wood.

'What?'

'Tomorrow!'

'But I'm a girl!'

The other three turned to Emma upon the sudden realisation that she would be sharing a dormitory with five sixteen year old boys.

'No, no, no! I won't let you!' James said furiously, stomping his feet. 'Wormtail's too scared and Moony is okay but the others are pigs!'

'Hello? I'm one of the others,' Sirius held up a hand.

'And you are a pig,' Regulus said calmly. 'Ask any of the girls that came up to me today. Besides, I'll be one of the others.'

'Um, I don't know if you noticed, but I am currently James,' Emma said. 'I was talking about James being in the Slytherin dorms. I know you love your Lily-flower, but you're a sixteen year old boy and I don't want you perving on my friends in the shower or something.'

'I won't!' James protested.

'Yes you will Prongs, we know you will,' Sirius laughed.