Caio gets up and wishes he could pee into the void. It would be so convenient! He has a water condenser for drinking! He'd never need to go to the bathroom until he needed to shower and do laundry! But alas. He doesn't really want to pee into the floor drain. He pokes his head out and makes meaningful eye contact with another couple boys who appear to be having the same thoughts and they all shuffle to the bathroom and Caio accepts being the last one to get to actually pee because he can't jockey for position very effectively. One of the other boys apparently did pee into the void and just needs to fill up his water jug, a big plastic thing.
Caio finds his homeroom slip only when he goes back to his room after that. 409. It's in the fourth section, approximately, but he doesn't see what multiplying by a hundred and adding nine does to improve its findability, since it isn't obviously next to anything except a stairwell and some maintenance-hatch-scarred walls. He goes there, looking nervously up the stairs, and joins the girl who's there already in investigating the furniture and vents and the inexplicable set of shelves on the far wall..