Sounds way overly complex for a high schooler to pull off. At least the OP sounded legitimate, the details didn't sound over the top.

I think you're underestimating motivated high schoolers.

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.

Not really. Sounds like this was class of '08, and at the time BackTrack would have been readily available and popular enough for a curious highschooler with a bit of computing background to find. As I recall etercap was built in and I wouldn't be at all surprised if there were tutorials for setting up scenarios almost exactly like what is described.

Even the ARP balancing thing is the kind of too-clever-by-a-half solution a naive youngin' would come up with since it would lead all the nodes thinking each other are the gateway and crushing the network with routing loops.

Maybe they hardcoded the real gateway's MAC Address.

They did not: https://news.ycombinator.com/item?id=28846569

Sounds like you hung out with the wrong kids in high school.

A couple friends and I pulled off some stunts of comparable non-digital complexity. (This was the 80s, schools didn't have networks.) They were more of the logistics and misdirection sort; for instance, having your own version of the printed graduation programs delivered, instead of the boring, official one.