Miguel is getting wound up about schools banning Skype. I'd really prefer that schools allowed its use, but it is important to understand the central issue here:
People like Skype because it works almost anywhere.
Skype works almost anywhere because it aggressively punches holes through whatever firewall is in its way.
The security professionals whose job it is to create and maintain those firewalls look darkly at this practice.
The underlying problems are that the large majority of schools use an operating system with a long and painful history of insecurity, and that a good chunk of the computers on the internet are compromised computers running some version of that same operating system, spewing out all sorts of attacks against their peers.
With current technology, it is hard to make a definitive argument against the security guys.
We need better technology.