An "onion router" is an Internet site that takes requests for web-pages and passes them onto other onion routers, and on to other onion routers, until one of them finally decides to fetch the page and pass it back through the layers of the onion until it reaches you. The traffic to the onion-routers is encrypted, which means that the school can’t see what you’re asking for, and the layers of the onion don’t know who they’re working for. There are millions of nodes—the program was set up by the US Office of Naval Research to help their people get around the censorware in countries like Syria and China, which means that it’s perfectly designed for operating in the confines of an average American high-school.
Tor works because the school has a finite blacklist of naughty addresses we aren’t to visit, and the addresses of the nodes change all the time—no way could the school keep track of them all.
There's a more complete overview, here
, but let's get on to installing Tor.