Fun thread. My first computer mischief experience was in "Computer Class" in first grade. The computers were running DOS 3.0 (IIRC) and the class was for teaching computer basics, which mainly consisted of "Introduction to programming" using Basic (with a gentle introduction to I/O, variable assignment, and flow control.) The way the class worked was: an assignment was printed out and handed out to all the students. You followed the steps and at the end of class, the teacher would look at your output to see if you did everything correctly. I finished these assignments pretty quickly, so I used the time to figure out how to exit the editor, find the "hidden" games stash, play them for a bit, and then get my work back up on my screen before the end of class. Pretty soon I was being asked by all the other students how to perform this feat on their own computers. I showed them, and so every time the teacher would leave the room, everyone started playing the games.

We also never got caught. Wild times in first grade, let me tell you.