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.
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.
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.