I agree in a sense, but I think the emphasis within any school should be on education. Even if we accept the fact that any expression through these mediums isn't private, teachers still have no authority to begin dictacting the do's and dont's of a students life. They are ultimately there to educate. The fact that they are permitted to use disciplinary action is nothing more than a tool used to maintain order in the classroom.These aren't simply personal life though, in many cases they are there for all to see and the ramifications of what they write can be significant. Twitter in particular is essentially an added dimension of the media as much as it is social networking.
If on the other hand there are things like personal attacks and defamation happening within the school through the likes of Facebook and Twitter or whatever, then I think the school has sufficient grounds to get involved.