from chris and harry's half baked senior prank ideas
jack all the silverware from the school cafeteria, and stick them into the football field. takes forever to get out
flour plus water = huge mess on football or baseball field with no damage to field
if there's a fountain around, fill it up with prop pirate ships and stage a pirate battle with notable local characters
Giant posters of tub girl www.tubgirl.com
would be a good nasty way to go.
and an easy one but will surely get to people is go to the pet store and get a bunch of crickets from the snake food section and put them in gatorade bottles. when you get to school open the caps to the bottles and drop them in the stairwells or in odd places of the school where people won't see them immediately and wait for the girls to freak out and the teachers to say "WHAT THE HELL IS THAT SOUND!?" we almost did this, but decided that our local pet store was too damn expensive to get the crickets from.