As a special education law practitioner, I believe this is a legal question.
You have presumably selected your son's psychologist because you trust her. You need to decide whether to follow her advice or not.
Assuming you do follow her advice, get her recommendations for your son in writing and provide the school principal with a copy. Ask for an IEP meeting to discuss the recommendations and your son's placement at different school/setting. Go to wrightslaw.com to learn what to do if the school stonewalls. Be prepared for the meeting, be professional, not emotional.
Consider filing a claim for the assault with the school district's human resources department. The school district carries liability insurance and can be sued for money damages. Talk to the police about criminal charges.
Consult a special education attorney or advocate if you need to. Your son is entitled to free appropriate public education that allows him to make progress. He has likely not made progress due to the environment at the school. In fact, he has been harmed.
Good luck with your continued advocacy for your son.
This is not a legal question. You need to decide what is best for your son. You have skilled and experienced expert opinion via your pediatrician and the recommended psychologist to assist you -- you don't need a legal analysis here.
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