Something I've done on nearly every background I've ever done (that no one's ever seen....in games I've never finished!) is to have some silhouetted objects in the foreground that the player can walk behind. Things like pipes, shelving, furniture that would make sense in a corner of said room, so on. I'm not sure how you could apply that here, but if the school is intended to look a little dysfunctional then maybe some cables hanging from the ceiling to inticate that lights may not work well. You could have a light out to create a little shadowing maybe?