On a school Mac I figured out using some command I could create a new admin account. I used this account to gain access to the school WiFi password and admin account password. I found out what a vpn was and brought my own laptop to us instead of the crappy ASUS Eee PC netbooks. The Vice Principal was not happy. They called my parents for a meeting ( I had a lot of issues with many IEP meetings). My parents were okay with it.

We would also pass around pirated GTA 3, GTA Vice City, Free versions of Minecraft, and Halo CE and run them off of USBs.

would put portable games on the public network share, apparently they didn't have logs of people putting stuff there

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…

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

We never had school computers :(

I used to goto cartoon websites and play unallowed computer games, nothing inappropriate, just spongbob stuff, which got me permanently banned from watching spongbob at home.

The school would keep using Internet Explore 6, I'd update it to iE 8. They didn't like that.

I'd also would install some toolbars, there was this one yahoo one, that added tabbed browsing to iE6. :)

Updating to iE8 on only one older computer caused problems, but the newer machines worked fine with iE8. ;)

I would install Firefox, Opera, Google Chrome and Apple's Safari for Windows web browsers. I enjoyed testing and find the best browsers back then.... Staff weren't to happy though.

I'd change desktop wallpaper to windows XP's Bliss, instead of the schools preset solid colour background.

As i got older, I learned how to write batch and vbs scripts to automate different things, so i wrote a tool to automatically login to some of my required school accounts, without me needing to manually type in my login credentials each time. Staff either didn't know or didn't mind this.

installing linux as dual boot, well it fixed the windows 7 that couldn't update lol

We installed Doom on a couple computers, this was in '97. Our computer teacher had absolutely no idea how we did it. Private schools were fun for running circles around the teachers.

Netbus 1.70 baby