2 stories that I thought fit into this thread, but aren't actually senior pranks:
1. So my dad took shop class when he was in high school. And their teacher was a total dick. He always told them that they weren't doing it right, and that they sucked at welding. Well, before they all left, the graduating shop class went out into the parking lot, and - with absolutely perfect welds, welded the shop teacher's car to the fence.
2. Another one with my dad. He used to work down on Queens Quay (for all you Torontonians), and they shared the building with its owner and his small business. The owner's business was renting out those massive ass forklifts for like industrial workplaces and shit. Anyway, over the time that my dad worked there, people often parked in this this other guy's (the owner of the forklift company - lets call him Steve) reserved spot. Each time, he got angry and upset, but people kept on doing it. Now just be known, that Steve was a very odd man, and came from a bit of a hick-ish area. Anyway, one day, some random parked in Steve's spot, and I guess Steve was just having a really bad day, but he decided to take one of his huge ass forklifts, and put this random's car on top of the roof of the building (which was like 6 stories high). At the end of the day, the guy comes outside, and sees his car on the roof and goes ballistic. He calls the cops, and when they arrive, he goes up to Steve's door, and knocks on it. When Steve opened the door, the guy was like "MY FUCKING CAR IS ON YOUR GOD DAMN ROOF!!!" all pissed and shit with the cops right there thinking they were gonna arrest Steve... my dad thought Steve was screwed over. Then, Steve went outside, acting all curious and shit, and then he replied "WHAT THE FUCK!?!?!? YOUR CAR IS ON MY FUCKING ROOF!!! YOU BETTER GET THAT FUCKING THING DOWN RIGHT NOW OR I'M GOING TO GET YOU IN SOME BIG FUCKING TROUBLE!!!" Now the cops were all confused and they started to be like "well it is his (Steve) property... and if you don't take it down soon we're going to have to charge you for tresspassing or w/e" and in the end, the random had to call another company to come with their forklifts, and he had to pay the entire process to get his car off of the roof. I'm pretty sure he never drove to work again...