In C++, teacher quit and we got a sub who didn't care, so a lot of people were playing unreal tournament. I use command prompt to run NET SEND and caused their game to minimize.
One kid caught on and so I taught him the numbering system for the computers. However, he instead ran NET SEND * You've been hacked! (or something like that). Every computer in the school got it and he was banned from all computer usage.
A teacher slipped on a french fry (UK: a chip) and so eating lunch in hallways was banned, despite the fact that everyone couldn't fit in cafeteria room.
In an effort to resolve people being late to class or hanging out in the halls too long, the principal placed herself and a desk in the middle of the hallway in order to get people to go to class despite the desk causing more congestion in walking traffic.
We started a fake club called The Future Entrepreneurs of the World a.k.a. The FEW. Everyone had a "elected position" in the organization, in order to place additional credentials in college applications.
One day it was 0 degrees outside. Me and a few other people filled up water bottles in order to pour it onto a 30 degree slope of sidewalk about 60 ft long. Our giant ice slide prompted the door that led to it to be locked on the following day.