These were the days when nothing had SSL, so you could just intercept and rewrite traffic!
My only requirement was: do no actual damage
It was implemented as a Debian live CD that you could drop into any school computer. It would boot up, then Ettercap would MITM the whole network by spoofing the router. It routed all HTTP traffic via Squid and a custom ICAP server that did the actual rewriting. If you removed the live CDs, the network just went back to normal within a couple of minutes.
Routing the whole school's network through one old Pentium machine wouldn't work though, so I figured out a way of doing distributed load balancing: it would do the ARP spoofing slowly and randomly. So, as you added more machines, it would just magically balance between them.
It worked great for about an hour then whole network mysteriously stopped working for the rest of the day. I left all the live CDs in the computers as a calling card.
Sorry, school network admins.
When I was in high school I was a huge Linux fan and had a side job as a network administrator for small companies in my town. I don't know if I would have gotten the "random ARP load balancing" idea, but overall it seems well within the knowledge admins of the days had about TCP/IP.
When I was between 15 and 17 or so, I wrote small HTTP, DNS servers etc. in C++ for fun (straightforward implementations and not better in any way, so in the end just learning exercises), and I definitely had friends who did similar things.