Of course you could sue the teacher and school but with no damages there are likely no recovery. The teacher is likely culpable for battery and assault providing minimal recovery. Most attorneys would not be interested because the case is not financially worth their time. If your child was not injured you should still file a claim with the school's insurance carrier to cover any out of pocket medical expenses.
Hope this gives you some insight into why the attorneys are not taking your case.
Best of luck.
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There should be someone at the district level who is responsible for handling parent complaints related to special education students. Filing a lawsuit may not be an option for you, but you can always file a formal complaint through special education channels or through the district's general grievance/complaint procedure. There most likely won't be any financial compensation for you in the end, but you will have spoken up on behalf of your son and maybe prevented this from happening to another student in the future.
Depending on your state's licensing procedures, you might also be able to complain to the state's educator certification dept. to have a flag placed on the teacher's teaching license or certificate.