I don't know if you've seen the people who planted a tree in there schools turf field, but planting a decorated tree (large tree) on campus would be pretty funny. Pretty hard though to.
-Fill all the toilets with clear jello
-get mosquito eggs and put them near the ventilation for some of the buildings
-graffiti the grass so it can wash off so you can't in too much trouble
-glue a door shut (a lot of doors)
-the thing where you let three animals loose and number them 1,2,4 so the people think there is a third.
-lots of bouncy balls
-buy a lot of clocks and set them all to go off at the same time and put them places
-take all the chairs out of a room
-get a universal remote and constantly turn tv's on and off
-giant pac man and ghost costume and run around the school
That's all i got.