Place active beehives throughout the school
The cricket gag is always decent... raleasing thousands of crickets free in the school so they chirp all day
stinkbombs... though I would suggest you stay out of the school for the rest of the day
Get a huge all out tackle football game going all the way across the commons/lunch room
Anybody say streaking?
as for my school, it seems most the seniors just got wasted and smelled of booze all day... don't think we did a senior prank.