I have a very similar story. In high school, our library was using a windows environment and through some luck, I discovered NET SEND or something like that. I figured out my friend's computer names and I started sending them messages. We eventually communicated this way even under the strict librarian and I eventually hatched a plan to annoy everyone. I put together a crappy batch file that iterated through every computers name and just mass sent messages but screwed up the iterator and it went forever. I think we had to restart all the computers but no one figured out it was me except my friends.

Miss those days and also miss playing soldat on those crappy PCs.