Alright, my school has a long tradition of seriously messing with each other, both internally & with exchange students (spending a semester at our school).
1) Let me espouse my love of dry ice: Freeze about 5 or 6 cans of shaving cream solid, and then chuck them into a small car (VW Bug, Ford Escort, &tc.). Ensure full watertight integrity, and the things will
FILL the car until it's pressurized
When they open it up, good times... but hard to get out of the seats.
2) Second love: dead animals. Everyone loves dead fish in the ceiling tiles, but crabs actually work better & smell worse (you can tell I'm living in Crab Town U.S.A). If you've got a campus with a lot of trees, dead squirrels will also be in abundance -- one of those on or in a pillow case will give a nice scare.
3) More sanitary but more logistically demanding: big trash bags full of leaves (this is the perfect time of year in the northern hemisphere). Take 7 or 8 bags to completely fill most dorm rooms, nearly to the ceiling, and it's a mess to clean up.
4) Classic harmless but annoying prank: bucket of water poised above the door so as to fall when the door's opened.
5) Winter season prank: spray hot water all over the snow above a door awning / overhang. When the door is opened & closed, the snow & ice will all avalanche off onto the person (this one takes finesse).
And, for seriously messing with someone -- this is a mean one, and will cost them bookoo bucks:
6) A cup of sugar in the gas tank. It'll caramelize in the engine & their car will die horribly.