1st ammendment on the web

I recently read an article on cnn.com about a girl who wrote derogatory comments about her school's principal. She was at first put on probation after the court determined she was a delinquent child but she appealed stating that her comments were protected political speech under both the state and federal constitutions because they dealt with school policy. The court found this to be the case and dropped the charges. Now, I ask was this the right decision? Should a student really be able to criticize an authority because she doesn't believe in their policy on body piercing? The answer, I believe, is yes but it does go to a certain extent. If all she did was criticize his policies then she is protected but if she, or anyone else, were to make derogatory comments about the principal himself then it's slander and they should be punished.

How do we go about catching and punishing people who write derogatory comments about others on the web? If we were to catch someone criticizing another person on their myspace page how can we prove it unless we catch them right then and there because otherwise they can just delete it.