Everyone's incessantly telling you what to do while at school. Parents and teachers are telling you what you must think, which friends you should and shouldn't spend time with, what you can wear, and even what you must eat. It's a long list. You'd swear you're in jail. You would expect that since you're pressured to go to class and all that stuff, they would at the very least be gracious enough let you browse whatever sites you need to when you are stuck there. Alas, that is not true. The interwebs are censored, many web sites are blocked. However, there are many ways to bypass the restrictions, and it's a lot easier than getting your parents to unschool you.
An unblocker is a kind of gateway that work between the computer you're on and the site you want to go to. The unblocker goes to the site you want on your behalf, accesses the page, and sends it on to you. So the site you want to access never sees your IP. So in effect you are browsing sites in an indirect way. School filters only think you are visiting the unblocker, not the site you actually want.
An alternative method of unblocking Facebook in school is to set up your own unblocker site. This is more complicated than using an unblocker but better in the long run because you're in control. How to do this is not in this guide, but if you search for "How to make your own proxy" you should find help. You can also earn money from letting people use it if you put ads on it.
Other ways to unblock Facebook include by using sites that translate pages, like Google Translate or Babelfish. You can also try Google's cached version of the site. However, you probably won't be able to to log in that way, so it's still better to use an unblocker for Facebook, though these tips are great for many other sites.