The Scholomance really, really hates it when kids room together.
It's an incentives thing. Outcasts and other socially-disadvantaged kids already have a harder time surviving at all than popular kids, enclavers and the like. If you start letting people room together, that's yet another way in which they get fucked over: all of the kids with a choice are going to room together, in threes and fours, cramped sure but still better for their survival than being alone, they can sleep in shifts and keep any mals out and that's a straight-up improvement over regular shields. Meanwhile the kids who don't have such social capital are going to stay alone, and be disproportionately targeted by everything that goes bump in the night. Spreading targets evenly is better for survival, and the Scholomance is playing nothing but a numbers game.
There's also graduation. The entire senior hall gets shuffled as the whole school spirals its floors down one level every year, and then it's locked out of access to the rest of the school and the only way they can go is down: to the graduation hall, where they have to run past the hordes of maleficaria that await. If seniors could just go to the freshmen's rooms that night, free of consequences, and not be forced to graduate, they would. One more year in the Scholomance means one more year to learn more spells, get more well-prepared, be more likely to survive out there. But there are not enough resources for that, not enough food and supplies. Everything is budgeted tightly, and having five rather than four cohorts of students in the school at the same time is just infeasible.
So it's for these and other similar reasons that the Scholomance really, really hates it when kids room together. Each student gets one room, full stop, and they have to be in said room from curfew start to curfew end. And what happens to students who flaunt this rule and decide to try to spend a night together anyway is they become a cautionary tale. When you get in at induction and the sophomores and juniors and seniors tell you all of the horror stories, that's typically enough to scare most kids straight. Not all, though, and every year you have a few hapless souls who decide to try it anyway. Their screams echo in the night as they bang on their colleagues' doors, asking for help, any help, until they are caught and dragged away by the maleficaria that the school has specifically sent their way, killed and eaten or perhaps just eaten without being killed first, depending on which level of bad luck they have on their last day of life.
Of course, these kinds of rules don't really apply very thoroughly when Scorpius Lake is involved. Not that the school isn't sending mal after mal his way, weakening its own wards just to make sure these kids learn in the most permanent way how bad an idea it is to spend the night together. It does that, with a vengeance. Mal after mal is broken against the shield of Lake's resolve, and more pragmatically the actual magical shields he has to renew every now and then. Yvette Villeneuve sleeps, perhaps not very peacefully, but at all, needing all the energy she can get to heal from the stab wound she got from Jack Westing last night. Scorpius wouldn't let her spend the night alone, not with said recent stab wound, not when she was about to be assailed by mals all night long anyway—they can smell the blood in the water, so to speak, and she would be low-hanging fruit. If mals are coming for her anyway, Scorpius reasoned, he might as well spend the night there and keep her from dying. Mal after mal, all night long, being killed and destroyed and pulled apart, each death fueling Scorpius's next spell, systematically and thoroughly.
By the last couple of hours of curfew, most mals who would come have come, and he gets stretches of even up to half an hour to rest. Not that he can sleep, obviously, not if he wants to stay alive, but he can at least sit and wait a bit, breathe more calmly. And as soon as the first bell rings in the morning, he flops onto the ground and breathes a sigh of relief. It's... over, finally.