That's - very sweet. He - don't move - really doesn't want to kill her; he already didn't but this helps. It gives him something to anchor on.
...It begins to feel like a possibility, that he could move and yet leave her living. Convenient, given that it will be suspicious in the extreme if he fails to respond. And yet - he can't seem to make himself do it, the mantra of stillness having apparently succeeded. He feels his response time slip into the seconds.
How about this: He'll say 'Not really,' in three-two-one, counted as thirty-second notes to a second's measure; three two one -
He'll say it after a full measure, counting sixteenths, how about; it really doesn't make sense, anyway, to frame something in powers of two and then use three of them; what nonsense; of course that wouldn't work: 1e&a2e&a3e&a4e&a -
Fine. He's going to count out four full human seconds, in their entirety, and after that he will have responded to the girl's question.
One. This is ridiculous, taking up this much time for a basic response. He feels like a fool. But his more-sensibly-timed attempts failed...
Two. Maybe part of the issue is that he's being overambitious in trying to speak? He'll downgrade to shaking his head no, perhaps; that might be the key. Refraining from using his breath means there's no chance of accidentally-on-purpose inhaling afterward. Just because social maneuvering is something that normally comes easily to him doesn't mean that it currently does. This sort of body language will be sufficiently communicative.
Three. Or should he nod? No, no, he's clearly not fine, not remotely, and... he doesn't want to lie to her, he realizes. Even if it would be a realistic way for a character to act, if Eugene the seventeen-year-old human who attends this school would lie and say he's fine, a basic social nicety that preserves his aloofness in spite of its obvious uncredibility - Eugene the hundred-and-four-year-old vampire doesn't want to. Some amount of hiding behind a persona is necessary, but this instance isn't.
Four. The music is so pretty, so intricate both in its construction and in its execution. The pieces of it are all so different from each other, but they have that in common. He wants to listen to it until he understands it, until he can guess where it's going...
He puts his hands on the table to make sure he has her attention, and shakes his head, minutely.
Success - and now he's proven to himself that he can move without killing her. He should, he concludes near-immediately, use this ability for the end of not pushing his luck any further. And now he's back to wanting to remain still, relatively; it wouldn't do to have gone through all this only to move visibly faster than a human can. He counts out two measures and with great deliberateness - slowly, slowly - gathers his effects, and does not lunge for the girl's neck, and walks out of the room. He's a musician, by God, and he will keep time. What kind of amateur speeds up against the written tempo - his feet will fall, perfectly, on the half notes, in the direction of the door, and not anywhere or anywhen else. His half notes, which are considerably shorter than those representing the girl's thoughts - but he's preserving the chance to hear more of that later exactly by leaving the room in a minimally-suspicious manner. Step step step step step step door open step step turn look at the girl, her throat, her flesh - DON'T do that - door close.