The teacher's conduct was inappropriate particularly if it was not preceded by some type of a verbal warning. I would recommend meeting with the principal to formally register your concerns and writing a letter describing the incident. You want this letter to go to the principal and the school district superintendent's office as substitute teachers are "at will" employees usually hired off of a list of qualified persons. While you may not get "justice" for your son's ill-treatment, enough complaints of this type will possibly prevent this individual from being hired as a substitute if not at the district level, then certainly at your son's school.
You may also want to follow up on finding out what the specific qualifications are for substitute teachers in your district. If your sense is that the standards are too low, you as a parent can use your experience to encourage the district to be more selective in its substitute hiring standards.
This is not a substitute for a consultation with an education or civil rights law attorney in your community.
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