So, schools adopting social media policies is becoming very fashionable these days (ridiculously so, and lots of smart folks can tell you why, but whatever). New York City Schools is the latest to apparently be considering it.
Here's the thing. I do not care all that much legally what you put in that policy, with one exception ... just don't ban anything. You cannot ban Facebook. You cannot ban Twitter. You cannot ban teachers from talking to kids outside of school. If the First Amendment says anything, it is that you can share your ideas without governmental interference when they have no legitimate reason to regulate. That a person is a teacher is not a legitimate reason to regulate all their speech, all the time. It isn't. Trust me.
To show you, let's get conservative just because that is how our current Court leans. Let's say a school board wanted to ban a teacher from religious speech (participating in this prayer social website, for instance). Would that fly with the current court? No freaking way. None. Facebook is not different. Why? Well, look at this site - the Hawaii Catholic Youth and Adult Ministry Facebook Group. Can we ban teachers from talking to students on that site? No, we can't. They have both an expression and free exercise right to do so. Thus, we cannot ban Facebook. We also cannot ban teachers from talking to students on Facebook. Bans do not work in this space. There is just far, far too much constitutional history on the other side of that argument and way, way too many different scenarios that would be banned all in one fell swoop.
Now, you can choose to block these things on your school Internet, that's fine. You can encourage responsibility. You can institute discipline measures for disruptions. You can, well ... be creative. But, banning teachers from using social media in anyway is a step to far, constitutionally speaking.