The best way to unblock Facebook at school is to use an unblocker. You can get onto any other blocked site this way. You'll find lists of unblocking sites at the end of this page.
What do unblockers do?
An unblocker is a kind of gateway that pass information between the computer you're on and the site you want to go to. The unblocker accesses the site in your place, accesses the page, and forwards it to you. So the site you want to be on only sees the unblocker's IP. So, basically you are surfing sites in an indirect way. School filters think you are just visiting the unblocker, not the destination site.Some tips when using an unblocker for Facebook
It's better to find an unblocker that hardly anyone knows about. This is because popular unblockers will get blocked faster. Unblockers that you find by doing a search will be blocked quickly because teachers can find them so easily. Even if you find a good one it's a good idea to have a few others you can try if your main one gets blocked.Another way to unblock Facebook at school is to make your own unblocker site. This is harder than using an unblocker but better in the long run because if they block it, you can make another. Explaining how to do this is not in this guide, do a search for "How to make your own proxy" you should find instructions. You can also make some money from this by putting ads on it.
Other ways to access Facebook are by using translator sites, like Google Translate or Babelfish. Also try Google's cached version of the site. Unfortunately, it's not possible to login that way, so you're better off using unblockers for Facebook, though these tips are good with most websites.
More info: How to unblock Facebook