In secondary school (gymnasiet in Swedish) I made a little exe that I put on the autostart on one of the public PCs. That exe made various weird behaviours for a few minutes (disabling the monitor, messing up the keyboard/mouse etc etc) before it restarted the PC. If, however, you held down a certain key combo, it would show a nice popup before quitting. So I always had a free PC to access every break cause no one wanted that PC :D