I'm gonna go with my own senior prank, cause I'm a smug fuck who loves talking about himself. This may be hard to describe, so do let me know if i've failed miserably at the human practice known as communication. So.....
I graduated in 2002 when my school was only 3 years old. It was under construction for the better part of the year and during the last month of school, the soccer post (goal...thingy) came down to re-level the field. Imagine a large soccer poll sitting by the side of the field, just ready to be taken for something evil. Now heres the hard part- the front doors of our school were pull, so push from the inside, and had these handles that were rectangular (as you'd expect to see on a school). They were all level to one another and spanned about 10 feet for the 4 doors we had. The pole was 12 feet. The seniors were out a week before everybody else, so on THEIR last day, I came to school an hour before it let out and slid the pole into and across all the door handles, meaning they could not be pushed open leaving everybody trapped inside. not only that, but when everybody did get out of class, they came running for the doors excitedly ready to bust out to greet summer vacation, only to find they were stuck in school. I wasn't there to see it, and I heard the janitor came out 2 minutes after everybody realized what the problem was, but the cops got called and a whole investigation got launched, so mission accomplished!!! They sent out letters to the parents saying the culprit could have gone to jail for a minimum of a year just for breaking some kind of fire code and possibly 2nd degree murder should anyone have been trampled, but they never found out it was me. I guess it was somewhat reckless and stupid, but damned if I don't hold the record for most people pissed off at a senior prank at my old school!