At school, you are forever being told what to do. Your parents tell you to go, your teachers tell you what to learn, what to eat, what to wear, who to hang out with (and who not to)... the list goes on. It's like prison, except you didn't commit a crime. You would expect that since you are forced to attend class and all these things, they would at least have the decency to allow you to browse whichever websites you will want when you are there. But, no. The internet is censored, a good deal of web sites are blocked. Luckily, there are a few ways to go around that, and it's a great deal more easy than convincing your parents to homeschool you.
Unblockers are like a gateway that work between the computer you're on and the site you want to go to. The unblocker visits the site you want in your place, downloads the page, and sends it off to you. So the site you want to access only sees the unblocker's IP. So, basically you are surfing sites indirectly. School firewalls think you are just accessing the unblocker, not the destination site.
An alternative way to unblock Facebook at school is to set up your own unblocker site. This is more advanced than using an unblocker but it pays off because you're in control. 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 also earn money from your unblocker if you put ads on it.
Other ways to get on Facebook include to use sites that translate pages, like Google Translate or Babelfish. You can also try Google's cached copy of the site. Although, you probably won't be able to to login that way, so it's still best to use unblockers for Facebook, though these tips are good with most websites.