You are always being ordered around at school. Your parents tell you to attend, your school staff tell you what to learn, what to eat, what to wear, who to be friends with... the list goes on. It's like prison, apart from the fact that you didn't commit a crime. You'd guess that since you are coerced to go there and all that stuff, they'd at the very least be decent enough allow you to browse whatever sites you will need as long as you are there. Alas, that is not what actually happens in reality. The interwebs are censored, and a good many of websites are blocked. Nevertheless, there are many ways to bypass that, and it's a lot more easy than getting your parents to unschool you.
Unblockers are like a gateway that pass information between your computer and the site you're trying to visit. The unblocker accesses the site in your place, 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 surfing websites in an indirect manner. Internet filters think you are just visiting the unblocker, not the real site.
Another way of unblocking Facebook at school is by making your own unblocker tool. This is harder than 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 search for "How to make your own proxy" you should find instructions. You can even make some money from your unblocker by putting ads on it.
Other ways to get on Facebook are by using translator sites, such as Google Translate or Babelfish. You can also try using Google's cache copy of the site. However, it's not possible to login using these methods, so it's still best to use unblockers for Facebook, though these tips work with many other sites.