Almost every computer in my school had some software on it that teachers could use to remotely view or control it. The software came in two parts, the "teacher" part and the "student" part.
Now, there was a three-part fuckup that led to entertainment:
For some reason, most of the teacher computers actually had the "student" software installed, the main exceptions being the IT teachers' computers and the computers that were more for office stuff.
The wifi password was weak, and some of the more computer-minded students managed to crack it. I had my own laptop that I'd sometimes bring in, and now I could connect to the school network.
There wasn't really any authentication on the software itself. As long as you were on the same network, the "teacher" software could mess around with any computer with the "student" software on it.
So naturally, once we figured this out, we started to mess around with other people's computers, including the teachers. We had one particularly lax science teacher who didn't really care what we did, so we Rickrolled him while he was connected to the projector. Also started playing flash games (I think Tanks and Bloons were the big ones at the time).
Also, another funny thing, while all the computers could only run authorised programs, the "Teacher" software could remotely start any program on those computers. So naturally, we started hosting Age of Empires II and Unreal Tournament games.