It depends more on the limits the school network has more than anything else.
Proxies from a website or program can bypass many things. But I worked for a school network for awhile, and the Admin was a smart guy, he went through the firewall settings meticulously and closed every port that was not needed. I remember while working on it 20 people called to asked why their MSN messenger didn't work... "Oops, guess I will open that port again."