this was done at my school recently to some girls on the team. the guys used milk though, instead, and put the cups EVERYWHERE. under beds, in closets, etc. 2 months later (last week) they were still finding the cups, which were now moldy, had bugs and stuff in 'em. classic.
also, just thought of one. a small creek runs through our school. every year, atleast one group of guys takes a mallard duck and puts it into a girls room for an hour or so, and lets it go buck wild. crazy stuff. duck poo everywhere.
one more that i just thought of: buy crickets from a pet store--the kind you're supposed to feed to live animals, and let them go inside someone's room.
oh wait, the best all time. freshman year, several of us took a dump into a pink cake-box. we carried the box to some dude's room (i didn't know who he was), b/c he and his roommate went camping for the weekend. we close the doors and windows and after like 5 minutes, the place reaked! there was a large pile of dirty laundry that was below his bunk bed. we put the box of poo on the floor, and covered it with his clothes. i don't want to imagine the smell of the room when they came in on sunday night! haha.