You think on it for a moment, and decide to tell your teacher everything, save for the contents of your meeting with the ninja. That covered a number of topics that either aren't relevant to everything that happened after, are secrets which aren't yours to tell, or touch upon sensitive information that you'd prefer to discuss with your sensei in a more private setting - ideally, one that doesn't have half a dozen strangers with (presumably) advanced kung fu training hanging about in ki-enhanced earshot.
"It started when we were on our way back to the arena, after meeting with Kasumi's family," you begin. You describe how you originally noted the supernaturally-dense shadows in that hallway, and how Altria - who gives you a wordless nod when you glance at her - sensed nothing unusual, due to her inherent magic resistance. You go into depth about the sensations you picked up through ki-scanning, particularly the nature of the older girl who you did not see, but who was either controlling or at least able to communicate with the shadow-creature without being attacked out of hand. The entity's ability to passively absorb small amounts of ki also gets a mention. You touch on when, how, and why you alerted Briar, rather than moving ahead, going around, or poking your nose into that room, and mention that Altria had her own mentor on speed-dial, but that you agreed to hold off on calling him until the situation got bad. Finally, you detail the unseen girl's emotional reaction to hearing a group of strangers show up outside her door, and how a moment of surprise and some less-than-ideal decisions on your part snowballed into her loosing the big walking shadow on all of you.
Lu-sensei listens to it all intently. Finally, he nods.
"Alright. Do you have anything to add, Miss Drake?"
"Only that I would prefer to wait for a chance to speak with my own guardian, before talking with the authorities, sir," she replies.
Your teacher nods. "Fair enough. From the sound of things, that shouldn't be too much longer."
Indeed, the sounds of the battle have gone from general havoc and an exchange of magical violence to a frustrated, inhuman bellowing and repeated hammering against something that rings like a huge gong. Glancing out the door, you can just make out a glowing dome of energy on the balcony, centered on the general area where you last saw the shadow-creature. The hemisphere flashes from translucence to brilliant blue opacity each time that deep ringing sound goes out, and after several of those impacts, you can see that the thing is getting smaller. You're not sure if Ambrose's spell is intended to banish the dark entity from your world, imprison it, or just crush it out of existence, but it seems to be well on its way to ending the fight.
"In the meantime," Lu-sensei continues, "if anybody other than Ambrose comes in here and tries to talk to you about this, tell them what you just told me - and you two," he adds, looking at you and Cordelia, "tell them that you want me present. Understood?"
"Yes, Lu-sensei," you reply quickly.
"Yes, sir," Cordelia says quietly, nodding. She hesitates, and then asks, "You won't be gone too long, will you?"
"No more than half an hour," the old man says firmly. "Will you be alright until then?"
Cordy nods, which isn't as reassuring as it should be. She ought to have said something like, "Are you suggesting that I'm not fine right now, buster?" or "Please. As if some walking shadow with zero fashion sense is going to get me down."
As Lu-sensei gets up and leaves the room, you consider your next move. Cordy seems responsive enough to talk, now, so the question is, what do you say to her?