Interesting on the First Amendment front considering a teacher or principal could be recognized as a public figure or even an involuntary one. Though, I believe there is specific laws (prolly judgemade) for defamation and libel concerning student blogging and social sites such as myspace.
Myspace=
anyway, there is proposal in Congress to ban access to those kinda sites on educational properties. Not relevent to this case but I hope they pass it. No parental supervision, a stomping ground for criminals and a waste of educational funds IMO.