Here are a few ideas, depending on the size of the school
Paint 1, 2, and 4 on pigs and let them loose, they'll never be able to find the third one hehe. It'd have to be a pretty big school though.
You could dump marbles down a stairway and watch people try to balance.
You could take all the garbage cans and garbage and make a mountain out of it, and add some dead animals and stuff too.
spray paint some choice words all over the school.
Just some stuff.