Recently in Nashville a parent planted a recorder on a child. Sure enough, the teacher was abusive to the kids, so with that evidence, I believe the teacher is now suspended pending an investigation. The problem with your case is that if this was a one time instance, it may not come up again.
Please note that my answering this question, does not in any way mean I represent you for this, or any other case. You need to seek that actual face-to-face advise of any attorney in your area who can advise you further as to your rights.
In this situation, I wouldn't give up and would keep pressing for answers/resolution to this incident. It doesn't matter (in my opinion), if it's a one-time deal or a repeat offense. If the allegation is true, then the teacher needs to understand the consequences of his/her behavior. Teachers are role models for our children. Yes, as adults, we all get frustrated, but I recommend that you keep asking questions, contact the school board again. And, if no answers are forthcoming, then you may want to consider contacting the entity in your state that licenses and regulates teachers to discuss this issue/see if there is anything else that you (as a parent or a group of parents) can do. I'm providing a link to that website.
Note: I am not licensed to practice law in your state. I provide this response for general, information purposes only. This is not to be construed as legal advice. Consult appropriate legal counsel in your area.
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