Ok, I'm old and this wasn't a computer prank but it's along the same lines.
I used to have a digital watch that functioned as a small universal remote. (It looked like an 80's calculator watch with tiny numbers.)
You did have to program it with the universal code for that brand, but my middle school had bought their TVs in bulk, so the ones permanently mounted in the rooms were all identical models.
I simply programmed my watch to that model, and I'd occasionally keep turning the TV on during a lesson. I did it fairly infrequently, and always in different classes so as not to give myself away.
I never got caught. Back then Tvs only went to channel 100-120ish without special equipment for satellites. If they went higher I would have LOVED to keep changing it to channel 666.