At school, everyone constantly wants to order you around. Parents tell you to go, your school staff tell you what to think, what to eat, what to wear, who to be friends with... the list goes on and on. It's like prison, except that you didn't commit a crime. You'd guess that since you are being coerced to go there and all that stuff, they would at least allow you to visit whichever web sites you'd need as long as you're imprisoned there. But, no. The internets are censored, and many sites are blocked. Even so, there are many ways to unblock websites, and it's very much more easy than convincing your parents to unschool you.
An unblocker is like a gateway that pass information between your computer and the site you're trying to visit. The unblocker accesses the site instead of you, loads the page, and forwards it to you. So the site you're trying to visit never sees your IP. So in effect you are browsing websites in an indirect way. Internet filters only see you accessing the unblocker, not the destination site.
An alternative method of unblocking Facebook at school is by setting up your own unblocker site. This is more complicated 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 too much effort to explain here, if you search for "How to make your own proxy" you should find instructions. You can also make some money from letting people use it through advertising.
Other ways to unblock Facebook are by using sites that translate pages, such as Google Translate or Babelfish. Also try Google's cached copy of the site. However, it won't let you to login that way, so it's still better to use unblockers for Facebook, however these tricks are good with many other sites.