I guess I missed the part where "Becoming A Fan" on Facebook was some sort of binding contract stating that you apparently agree with every comment posted in the group. To fix your analogy, becoming a fan is like coming into the classroom every day and reading what's on the blackboard/paper on the wall and adding your name to the list of "People who've read this."
Having been in a similar situation as this before (except my friends and I were suspended for it), I'd be interested to see what comments the superintendent felt were libelous. IANAL but from what I understand opinion, so long as it is not expressed as fact, is not libel. "I think Mr. Teacher is a knob-gobbling pedo" is hurtful and mean, but its an opinion and would likely not hold up in court. "Mr. Teacher likes little boys" probably falls into libel. But, that libel only applies to the person making the statement. Making that statement surrounded by a group of ten people does not in fact make those 10 people guilty of libel (or in this case slander). As far as I can tell with regards to the law, there's no "huge difference." Making a verbal statement over lunch is slander, writing it down is libel. The only difference is the context of the statement.
Also, last I checked, there's no asterisk by freedom of speech in the Constitution. There's no rider that says none of this applies until you're 18. Schools would like to think differently, and most of the time, everybody gives in because 1) somehow "it's for the good of the children." and 2) wasting a few afternoons sitting at school is cheaper than going to court to prove a point.
I'm kind of new around here, but I would myself be really leery about posting anything like "I hold patent 12345," especially when, without reading the patent, you appear to have described a webcam, which according to the almighty Wikipedia, has been around since 1991.