You could always kill a deer and put its head in one of the restroom toilets. This was done at my friend's school.
At my school a new building was being constructed on campus at the time of graduation. The night before the big day, someone got a paint roller with a huge pole and painted a two-story cock on the insulation of the building. Across the bottom floor was written "2006," with the 00 being the balls for the penis. "Got balls?" was painted in smaller letters next to the year. The janitors tried to cover it up in blue paint before all the parents showed up, but only succeeded in giving it blue balls.