How did you mess with school computers?

I remember in early secondary school there was a weird desktop that would briefly flash during the login process. A friend and I decided to keep logging out and in and furiously click around to see if we could access it, one of the times we did it and that desktop session stayed, there wasn't anything special about it besides a blank windows command prompt, we closed it...

Cut to the school computer systems being down for over a day and noone knowing why, felt pretty scared of being found out over the following week!