We had one Sophomore year where the Seniors handed out flyers for a pool party (that was to be on the school's roof) to all the freshman. At least half (more like 3/4) of the freshmen came in with swim trunks. Girls wore bikinis under regular clothing. Needless to say, there was no pool party. The joke still surfaces around school every now and then.
We had one last year where a donated car (some senior was buying anew) was sawed in half, then put around the flagpole out front. An assistant principal's car was put in the building right in front of a staircase ( the main principal is awesome, the assistant, not so much). Freshmen were water-ballooned. A week in advance, however it got leaked that the water balloon thing was happening. Senior prank day, several freshmen girls were wearing white tees. That's our county.
Edit: Oddly enough, Dicho, everyone walked last year, including people who participated.