Lots of schools use a program called Bess (http://www.securecomputing.com/index.cfm?skey=1209) to control the sites that you can surf to on school computers. Bess has one flaw in it that makes it really easy to bypass... it doesn't filter sites that use SSL certificates (the URL starts with https instead of http). So to bypass Bess, all you have to do is use a proxy site that makes use of SSL. One such proxy site can be found here:

https://vtunnel.com/

If you use vtunnel, you'll be able to surf anyplace you like even if they have Bess installed to block traffic.