I don't remember this exactly but when I was around 10 years old (circa 2007), me and my friends were playing around this ".bat" file that you create using notepad with a specific line which I forgot but essentially restarts your PC when you run the bat file.

We had some laughs during computer class.

During student council meeting, I had the chance to use the teacher/advisor's PC and of course tried this .bat thing for some laughs. Unfortunately this PC was older or something because when I ran the .bat file it didn't just restart the PC but ran into a significant error (I think some important files got deleted). Good thing no one noticed I tinkered with the PC, because the teacher was flustered.

In 9th grade (1984), I had a typing class using IBM PC Jrs. I made a quick and very simple breakout game in BASIC one period and distributed to the rest of the class.

I had GLTron and Pocket Tanks installed on a flash drive, so my friends and I would just play games whenever we had free time.
I also found a couple fun network based utilities in Windows, and all the computers in the district were on the same network. I was messing with NetSpend one day and managed to accidentally send a message to every computer in the school, which then all promptly crashed for some reason.
I also had fun messing with the netchat application. My mom worked at another school in the district, and one time I arranged for her to open netchat at a specific time while I was in computer lab, so I could connect to her computer from a lab computer and we could chat back and forth.

We installed Doom (1 or 2, I don’t remember) in an invisible folder and played via the 10Base-2 network. Those were the 90ies…

Ok, I'm old and this wasn't a computer prank but it's along the same lines.

I used to have a digital watch that functioned as a small universal remote. (It looked like an 80's calculator watch with tiny numbers.)

You did have to program it with the universal code for that brand, but my middle school had bought their TVs in bulk, so the ones permanently mounted in the rooms were all identical models.

I simply programmed my watch to that model, and I'd occasionally keep turning the TV on during a lesson. I did it fairly infrequently, and always in different classes so as not to give myself away.

I never got caught. Back then Tvs only went to channel 100-120ish without special equipment for satellites. If they went higher I would have LOVED to keep changing it to channel 666 to freak people out.

All of the other people here have such cool stories to share, while I am here, who just changed the default browser of a few computers...

Replaced the Windows 95 boot screen with an exact copy where a single black square was changed to red.

Not neccesarily the computers but I used to have a phone with an IR blaster on it and had some fun with the projectors doing that

Install NixOS on them 😈

I taught to 2-3 naughty friends how to wipe something in the C:\ drive, some windows folder or something like that, and they did it in some Pentium PCs.

The teachers started looking PC by PC without knowing wtf was going on in the middle of the class.

They spent a few afternoon doing kind of community work at the college as punishment.

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.