As a teacher I can see why these might be nifty to have--"Your kid screws around in class too much, doesn't pay attention, etc. Here, watch."
I can also see the other side of that coin--"The teacher picks on me, puts me down, and yells too much. Here, watch."
I've been putting some thought into a topic lately: why do I support the legalized videotaping of police officers in the performance of their duties, but not the recording of teachers in the performance of their duties? It's easy to let teachers off the hook and say that we can't lock people up or cause them to be fined, but given the right parents we can certainly make a child's life none-too-pleasant. We're not armed, but we're an authority figure that's relatively respected in society. So what's the difference?
I'm also not willing to let teachers off the hook by claiming that most schools don't allow students to use that type of electronic equipment during school--imagine the cheating that could go on! That doesn't address the point of actually being recorded.
Is it really nothing more than "I might say something that I shouldn't, and I don't want it to be recorded?" Or is there some other principle that I know is there but on which I just can't put my finger?