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.

"Net send" message to every windows PC in the school. IT was not happy about that. Guess they should've disabled that exploit. I think it was before windows xp had (SP2?) disabled it by default.

In college we had courses on Linux and we were able to SSH on other students' computers. First I used innocuous commands that ejected the optical drive or that enabled the screensaver.

But unfortunately it escalated quickly and soon every student would mess with each other by shutting down the computers...

Not in school.

But i have written a .bat scipt in notepad on an unlocked laptop in a store

Then added the script into the startup folder.

The script would restart the pc after 30 seconds.

shutdown -r -t 30

(R for restart, t for time, 30 means 30 seconds) its harmless.. but also super frustrating

Write it in a notepad file. Save it as a .bat file

Highschool had filters to prevent students from visiting certain (most) sites, but for some reason a browser created using Visual Basic Studio's browser template worked just fine. At least I think that's what it was called. It's been a minute.

Put some VB script (I think) that opened and closed the CD-ROM 50 times inside a startup folder. Did it on all computers. Also put a batch file there that shuts down the computer one second after logging in on all teacher computers.

And last but not least, I created a phishing Facebook page, opened it on some browsers in school, rewrote the URL to a Facebook one (without pressing enter) and left it there, collected some passwords.

Edit: Also installed Ubuntu (dual booted) on the computer I usually used.

Edit2: Disabled the tracking software for a computer I used. Damn, it's all coming back to me! Good times.

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.

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…

Write a script to create an neverending amount of files. Piratinh games. Trick teachers into giving us their passwords. I could go on...

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

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.

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...

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.

This was back in the 90s... we figured out a simple way to make 'empty' files using the spacebar ascii and qbasic. We'd have a simple interface, flashing cursor, and you'd type in a number; it would then create an 'empty' file of that many MB.

Of course being 14 yr old little shits, we wondered, how big can the file be? Someone created a file big enough that first it filled the student partition; then the teacher partition; then the temp partition; then the system partition, at which point the entire network slowed to a transfer speed of a couple of bytes. When we realised we could do this, it was happening several times a week.

After that, anyone caught with a blank screen and flashing cursor got in deep shit. They deleted the attrib command so we couldn't un-read-only / un-hidden things, but we just copied it from our own DOS at home and brought it in on a floppy drive