First, as a practical matter, you may want to (a) block all e-mail from this sender, if that's permissible -- you didn't mention whether this is a private e-mail address over which you have complete control or whether it's a school e-mail address for which you may have to follow school rules; and (b) consult with your teachers union rep -- the union, more than the school system itself, may be able to start the ball rolling with practical assistance, getting the school to take some sort of ameliorative action.
Legally, you may have the makings of a defamation action -- depending on the particulars (you understandably haven't provided much in the way of specifics here) you conceivably could have the basis for a claim against this parent for libel with respect to the written statements if the parent disseminated them to others beyond you, and/or for slander if the parent spoke the defamatory statements to others as well. For this, you'd obviously have to consult a lawyer in your area, preferably one who has experience both with school legal issues as well as the law of defamation (libel and slander).
Please note that I am not licensed to practice law in your state, and I practice only in NY. This response is provided for general educational purposes only, and no attorney-client relationship has been formed. It should not be relied upon as legal advice. The response given is based upon the limited facts provided by the person asking the question. To the extent additional or different facts exist, the response might possibly change. The response given is not intended to create, nor does it create an ongoing duty to respond to questions. As they say, "Your Mileage May Vary." Best of luck.
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