This is not an easy question to answer without knowing all the facts involved such as what events lead up to the situation, was it an “accident,” and if there was a mark left on your son’s neck. There is no reason why a teacher should ever have to grab a student by the neck. If your son was out of control, then the teacher should have been trained to know that he should call for help and get the principal or the counselor to help remove your child from the room.
As far as taking this to the next level, we can not give legal advice. Your principal or another administrator should have addressed the situation and disciplined the teacher. There should be a protocol in place such as when the school is required to contact the police or Child Protective Services. As a parent, if you feel the situation warrants a police or child abuse investigation, it is your right to contact them.
Placing the legal issues aside, your son’s emotional health should be your main priority. If he feels afraid to go back to school to face his teacher, then you should be able to talk to the school counselor or school social worker for guidance. If the teacher continues to be irresponsible and your son is affected again, then you may decide to take a legal course of action.