Well, back in the day the image file for the boot up screen and shut down screen was easy to find and change. I made some screens images that stated that the computer is being destroyed or they messed something up and the computer memory was wiped.
I feel like this is minor compared to what I've been reading on here.
I recreated the "IT IS SAFE TO TURN OFF THIS COMPUTER" image in paint and full screened it to trick the teacher into turning it off prematurely. Similarly simple, but hey, young me had fun!