You don't. It's as simple as that. 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.
One thing you COULD do with your spare time is learn how to express yourself using English.