How to get into blocked websites on school Wi-Fi?

I can't get into facebook on my iphone as it says it is blocked. My iphone is jailbroken so i was wondering is there any tweak or something i can download to bypass the firewall or whatevers its called

Bypassing a school firewall like that is a felony crime in all 50 states of the US. It is a form of hacking and you can be suspended from school for doing it. You could also be fined or even jailed.

Anytime you bypass a firewall like that, you open that entire network up to a virus or hacker attack. Should someone attack through your connection, you could be criminally and financially liable for any damage that they do. You could find yourself sued for lots of money. (Judgments are usually in the millions of dollars.)

I work in the IT security field for a living, and see this happen to people on a daily basis. There's nothing on Facebook so urgent that it can't wait a couple of hours until you are out of school and can legally access it. Stop trying to commit a crime.

You can Google for "facebook proxy" or "unblock facebook"

I know someone who did that and was dis-granted Internet access throughout the whole school grounds and was fined

You can easily use proxy or VPN to bypass the firewall.
If you like to use proxy www.hidemyass.com is a good proxy site.