In Michigan it is illegal to record audio without consent from all parties. It's not clear how these laws would apply to a system like this, but it could potentially mean trouble if someone tries to challenge the audio capability.
I don't think that there will be any successful challenges from the student body though, unless teachers have changed since I was there...
These laws are more generally 'eavesdropping' laws and although usually referenced in the context of making some electronic copy of a conversation, they equally apply to simple interception and disclosure when a party has a reasonable expectation of privacy.
And as this ex-4th grader can attest, in class at least, you are legally subject to interception and disclosure even when you think you have a reasonable expectation of privacy. Though had I thought of it at the time, I'm sure I would have at least offered the law up as a defense, right before my handwritten and carefully folded and private declaration was both 'intercepted' and 'disclosed' to my jeering classmates.