Jayne.
It's simple: you don't. The PC belongs to the school, not you.
If they want you not to use the PC in specific ways, respect their wishes about what they want done with their property.
I've seen first hand the damage that can be done by some pillock trying to be clever - "Oh, it'll be alright, I KNOW this site is safe." That was just before he copped a nasty trojan that tried to dump several GB across our corporate network.
The stupid pillock doesn't work for us any more. Or anywhere else that he'd have acces to a PC, I sincerely hope.
Facebook is a pretty bad place for picking up nasties unless you are pretty careful.
Stats from social networking safety apps suggest that one in five items on the news feeds of Facebook users lead to malicious content.
http://www.theregister.co.uk/2010/11/24/facebook_malware_survey/
http://www.bbc.co.uk/news/technology-11827856
http://www.f-secure.com/weblog/archives/00001517.html
http://www.theregister.co.uk/2010/01/11/facebook_charging_rumour_malfeasance/
THAT's why the person who owns the PC or the network doesn't want you screwing with his property (you know, stuff that's not yours).
One thing you COULD do with your spare time is learn how to express yourself using English.
A Grownup, January 2011