The person inside the room does not come out that night.

He doesn't keep watch literally all night, that would be silly. He goes back to his dormitory and has a reasonable amount of sleep and wakes up early and goes looking for Kerotiru again.

Eventually he hears window noises.

If he's climbing out the window, that's a sign that he has something resembling the correct amount of respect for Tassairen's ability to hurt him.

Tassairen lets him climb out the window, and then goes to be the first kid who shows up to breakfast.

Kerotiru: does not show up to breakfast.

Tassairen goes about his daily routine, makes sure to be the first to show up to every meal, and patiently waits for an opportunity.

Does "spontaneous unconsciousness" qualify as an opportunity? Probably not.

...it sure doesn't!

He wakes up glued to the dormitory ceiling with a note reading, "Nice try, kid, but you're out of your league; let it go and you won't get hurt" pinned to his shirt.

He unpins the note, reads it, burns it to ash, and gets himself disentangled from the ceiling.

All right. Stealth magic is evidently something that exists, and he doesn't know any yet. That puts him at a disadvantage. He needs to be clever about this.

The older favourites' rooms have magical protections on the doors, he knows that much already. How feasible is a 'quietly break into his room in the middle of the night' plan? He investigates his options.

Quiet, theoretically possible, if he can figure out how to fuck up the lock spell well enough; discreet not really, it would be incredibly obvious that the lock was fucked with after.

Oh, he has no intention of leaving anyone in doubt about whether and by whom Kerotiru has been fucked with.

He does intensive practice with picking apart spells for a week, until he's sure he has it. Then he breaks into Kerotiru's room in the middle of the night.

Kerotiru: Is asleep.

But he'd probably have a hidden spell guarding him, so now is not the time to walk right up to his bed. Tassairen spends a while carefully examining the magical fabric in the room first, to see if he can spot anything like that.

Yyyyyep. Some kind of ellipse, about three meters from the bed on average.

He doesn't know exactly what it does and he doesn't fully trust his ability to take it apart, but he does think he can work from outside its radius. He gets as close to the bed as he can while remaining a safe distance away from the mystery spell, and then he throws his pain spell at Kerotiru.

There is a blinding flash of light.

Then he will be holding that pain spell on Kerotiru for as long as it takes his vision to clear, and paying close attention to the magical fabric in the meantime.

The circle is much easier to see once it's gone off on him; maintaining the pain spell across it isn't setting it off again but if he tries to put anything else over the barrier it'll go off again.

Hmmmmm. Can he maintain the pain spell and work on picking apart the circle at the same time? Maintaining the pain spell is definitely the more important of the two, but if he can have both...

He can try, but it makes picking apart the circle significantly more difficult, and it'll go off if he picks at it and isn't careful enough.

He makes careful gradual progress while he waits for his vision to clear.

His vision clears significantly before he gets very far with the circle.

Hmm. And once he's blinked out the last spots and adjusted to the darkness again, it has been a few minutes...

He lets the pain spell go.

The boy on the bed doesn't move.