i have a theory but need to have some info first did the doors have some sort of glass on them? like you could see in the room almost like classrooms but not a full glass window like a small half window where you could see inside the door room to know that there is only 1 bed or 2 beds. if so he could have been looking in the windows at all the rooms to find one that was a singular room. he chose the 3rd floor because maybe he jumped out the window thus why the window was open? it would have been easier for him to jump out then risk being cought by going back down the stairs. not sure how high the building was and if jumping from the 3rd floor would have caused death or minor damages to the person jumping?
I agree. Could the person have entered and exited through the window? Did this boy have a secret life/people that no one knew about? It's strange, not sharing information for a religious belief or polygraph came up with nothing? Maybe so. How far did they extend their interview outside the school?