Well, to put it in perspective, lets look at my high school's district that uses an industrial-strength connection.
You have thousands of people running on a connection, so lets say you have a 100 mbs connection, well, with 100 people using it at once you'll technically only have a 1mbs connection. (or maybe I'm thinking the way phones work)
Even on a connection that powerful, and ignoring the fact that there are so many people connected at once causing it to slow down, think of memory.
any kind of game takes up a lot of memory in online play from the internet connection. I'd guess a server that strong has about, ohh... lets say 1 terabyte of data.
Now, think of how one movie can be as much as 10 gigs, and how a game can take up 10 gigs of memory with a few hours of play.
Now, multiply that by thousands of students. NOW realize that the gig cap for some of the best connections in the U.S. is around 350 gigs then they begin to charge you out the ass for extra services.
Yes, it sucks, but if you want public internet, you have to make some sacrifices to accomplish it. The school could always charge students to use services such as video streaming/online gaming to cover the extra costs, but overall its easier to just say "no."
EDIT: Also, to put it in simpler terms: Their school, their internet, their rules. Deal with it