They never discovered the rats we actually had installed on all their computers. They were still there years after I left, until they upgraded the computers.
We (a few friends and I) were basically the Novell Netware admins because our school didn't have a full-time computer tech and none of the teachers knew anything about it. We convinced them to let us help them in exchange for letting us read the manuals and tinker around, and we actually did help with problems and troubleshooting, mostly printing if I remember.
They never figured out it meant we also had access to the teacher-only folders though.
Back when I was a highschool freshman (2004-05) I wrote a batch script that would fire off net sends to everyone in the computer lab in rapid succession in an infinite loop, then just sort of left it on a shared drive with a conspicuous name. Sure enough, a few days later, someone ran it out of curiosity and got in trouble, but of course the file had my username in the metadata, and my computer teacher was like “Chris, you knew what you were doing, don’t do this again.”
It was the kind of “good clean fun” sort of prank that doesn’t get you in hot water or suspended, but was hilarious to watch play out.
Edit: Just re-read and saw that your friend got expelled for doing basically the same thing I did. That sucks. I’ll note that I went to an IT-focused votech school, so I think a lot of folks had a better sense of perspective as to how serious net send pranks actually were in the grand scheme of things.
We just used it for a place to run scorched earth from on the whole network without having to download it to our home folders where it'd get noticed and punished