release 3 pigs into the school, each branded with a number 1,2,4
get a few thousand paper cups and a fuckton of water and people to help, fill them and place them all over the cafeteria or gym floor
take apart a car and hoist each part onto the roof of your school then put it back together
get a really loud mp3 player dock and a cheap mp3 player ($25), hide it somewhere (duct tape it to the cafeteria ceiling if possible) then put some annoying-as-hell song on it and turn it up to max volume
draw penises (obligatory)
get two guys to get in pac-man and blinky (ghost) costumes and have them run around in the library
at graduation when each kid goes up to shake the principal's hand they each give him a marble, after 10 minutes the floor will be covered in them
take a few thousand plastic forks and make them in the shape of 2011 in the front lawn (for bonus points take the picture on google earth in a year and mail it to the principal)
i'm out of ideas, sorry
don't do anything too destructive or anything the janitor will get overly pissed at