Well, you can get industrial size bags of jello and dump them in the pool....on a smaller scale, you can put it in the toilets the night before....just make sure to mix it up. Oh yeah, you can put"things" in it...kinda like how people put banana's and stuff in it...you get the idea.
Otherwise, mounting furniture to the ceiling always goes over well.
One year one of the seniors dressed up like a gorilla and ran around the school, into and out of every classroom, throwing things around and flipping things over. Hilarious. He must have made it around half the class rooms before he got busted.
