You're incessantly being ordered around while in school. Your parents tell you to go, the school staff tell you what to write, what to eat, what to wear, who to go out with (and who not to)... the list goes on and on. You'd swear you're in some kind of prison building or something. You'd guess that since you are forced to attend and all these things, they'd at the very least be decent enough permit you to browse whichever websites you'd need to as long as you are held captive there. But, no. The interwebs are censored, and a good deal of sites are being blocked. Fortunately, there are a couple of ways of going around that, and it's very much more easy than convincing your parents to unschool you.
An unblocker is like a gateway that work between the computer you're on and the site you're trying to visit. The unblocker goes to the site you want instead of you, loads the page, and sends it on to you. So the site you're trying to visit only sees the unblocker's IP. So actually you are surfing sites indirectly. Internet filters think you are just visiting the unblocker, not the destination site.
Another way to unblock Facebook in school is by setting up your own unblocker site. This is more advanced than using someone else's unblocker but it's a lot nicer to have because you can make more of them. How to do this is not in this guide, if you do a search for "How to make your own proxy" you should find some guides. You can even make money from this if you put ads on it.
Some other ways to unblock Facebook include to use translator sites, such as Google Translate or Babelfish. Also try Google's cached copy of the site. Although, it's not possible to login that way, so it's still better to use an unblocker for Facebook, however these tricks work with most websites.