Um, I don't think it's the same thing at all. At a water cooler, you can see your audience. You are saying things that are for that audience alone. And when you are done, there is no record of what you said.
With Facebook, you are publishing content. It is written down. It is recorded, potentially permanently. Because you can't see the audience, you don't know who they are. Facebook is much more like printing up a what a posters and plastering them up. With your name on them. Even if you put them only where your "friends" hang out, other people can see them if they wander over. Your friends can move the posters around to other locations. Your friends can make copies of the posters and post them where they want.
There is a huge difference between writing something on Facebook and telling the crowd around the cooler.
You are aware that Facebook has privacy controls, right? Your comparison of putting up posters is completely unfounded, unless the kid is stupid enough to have his profile entirely public.
Even if that is the case, though, Facebook is no different from the water cooler. If you say something to friends around the water cooler, one of them could repeat it to the boss, and you'd be facing the same kind of consequences. Or someone could overhear you. Or someone could have planted a recorder and it would be permanent.
The fact is, just because it's written down doesn't mean it carries any more or less weight. If I make a threatening phone call or send a threatening letter, they're both harassment. The medium is mostly irrelevant... it's the content that matters.
In this case, it was a comment made off school grounds, outside school hours, about a teacher. If it was an ongoing thing, I could see grounds for libel and suspension... but a one-off incident? Hardly anything to get worked up over...