Everyone's forever telling you what to do at school. Your parents and teachers are insisting on what you ought to write, which friends you ought to and shouldn't be friends with, what you must wear, and even what you ought to eat. It's a long list. You'd be forgiven for feeling as if you're in a prison facility. You'd expect that since you are coerced to go there and all these things, they'd at least be nice enough permit you to browse whichever web sites you'd like when you are held captive there. But, no. The interwebs are censored, a great many of sites are blocked. Luckily, there are a couple of ways to bypass the restrictions, and it's a great deal easier than convincing your parents to homeschool you.
Unblockers are like a gateway that sit between your computer and the site you're visiting. The unblocker visits the site you want on your behalf, downloads the page, and forwards it to you. So the site you want to access never sees your IP. So, basically you are visiting sites in an indirect manner. School filters think you are just accessing the unblocker, not the site you actually want.
Another way to unblock Facebook at school is by setting up your own unblocker tool. This is harder than just using an unblocker but it's a lot nicer to have because you're in control. Explaining how to do this is too much effort to explain here, if you do a search for "How to make your own proxy" you should find some guides. You can even make some money from your unblocker if you put ads on it.
Some other ways to unblock Facebook are to use translator sites, such as Google Translate or Babelfish. Also try using Google's cache copy of the site. Although, it won't let you to login that way, so you're better off using unblockers for Facebook, however these tricks are good with most other sites.