Our school used to have a central windows server host and virtual environments for each student seat. They all had only a monitor mouse and keyboard that connected to the server using a username (all started with stu and then the number of the seat) but had no password.

A buddy of mine then went ahead and made a .bat script that somehow simultaneously tried to connect to all student seats, resulting in each of the screens blacking out one by one for a while, then going back to normal.

I ran it for shits and giggles at the end of class, and the teacher saw it, didin't understand what happened initially, got really angry and walked into a few chairs tripping up trying to catch us, and then took us to write a report with the school secretary. I love this teacher, this was one of the funniest moments in school.