As a school based tech, you have to understand that there are reasons for this. Not only does it put the school in a liable position to let kids use IM clients and proxies at school (unless the school has a legitimate reason to have this solution), it can get you in trouble as well since you're trying to bypass mechanisms that the school is using to prevent this. Schools have to adhere to laws set in place by the government, otherwise districts can lose funding from the government if a law was broken (this is a constant worry in our schools district now, especially with budget cuts). This is not to say it's a law specifically restricting IM or proxies, but it can put people (specifically students) at risk which would be a major liability for the schools and the district.
The only thing I can tell you to do is if you have some sort of cell phone plan that allows for data usage, to use that. This way you wouldn't be getting yourself in trouble by breaking such mechanisms that the school has put in place, and in turn it won't be a liability towards the school or district as well.
I know that in some school districts, forms are sent by the school to parents and students that deal with an "acceptable use policy" for school network/internet usage which must be signed and returned to the school. If this was the case in your school and you're trying to bypass these mechanisms, then you (and possibly your parents) could be liable since you (or they) agreed to be liable to what was signed.
If you're smart enough to understand about proxies and bypassing filters, then you should be able to understand clearly what I've just typed.
Just some friendly advice...
