The best prank I ever pulled was in the 10th grade when I figured out how the intercom system at my school worked. Through a friend who broadcast announcements in the morning we figured out that it was basically a closed-circuit telephone line, local to the school only, that had been modernized a little in this millennia with a computer to control which announcements went to which room. Didn't even use VoIP or anything. Originally it used an operating room not dissimilar to how old phone lines worked (the school was built in the 1920s) We came to the conclusion that there were some rooms in the school that for whatever reason would only go to other rooms while others would by default go to every room in the school. An example of this was most rooms could only talk to the main office while by default the main office was broadcast to every room. I think the main office had some sort of selector to talk to certain rooms at once but for the prank understanding this part was not really necessary.
Eventually we figured out there were some unused rooms in the school that had the intercom system installed but the speakers and microphones removed and by default for some reason these were treated exactly like the main office! Some juryrigging enabled us to connect a computer (they had plugs in the walls like what you would connect a land line to, it was an alternative to having to use the microphone installed in the ceiling) and we tested it a few times outside school hours with short test tones and the like. The best we ever did during school hours was bombarding everyone with
this for several glorious minutes. Those in the know thought it was funny as hell. The school must have figured out someone was screwing with it because next year all the phone outlets in the unused rooms had been replaced with metal plates. lol
The computers in the school were a whole other mess. I used a workaround to the security (using task manager of all things!) to overclock the GPU with the goal of being able to make GTA somewhat playable (the computers sucked from the get go, I don't know why I thought I could do that without melting the GPU) This led to rather interesting conversations with the computer maintenance department when I could no longer get it to boot.