It is very hard to answer when you don't say what you have done so far: did you call the cops? If not, that would be the first step. Also, practically every time we get one of these questions, the parent later comments that the child has special needs (very germane to put in your original question). If that is the case, hire an educational rights attorney to deal with the school because many don't have training or proper staff to deal with these children and often times the teacher's response is that she was trying to save the kid from hurting himself or others. The attorney can get the maximum amount of benefits, training and accommodations for your child. You can consult with a personal injury attorney for a civil lawsuit--he/she can assess if you have a case--you don't provide enough facts here. On the criminal front, report to cops and see if DA is interested in pursuing. Obviously, file a formal complaint with the school and district. Again, without saying what the teacher's explanation was, the steps you've taken already etc. hard to assess.
We do not have an attorney-client relationship. I am not your lawyer. The statements I have made do not constitute legal advice. Any statements I have made are based upon the very limited facts you have presented, and under the premise that you will consult with a local attorney. This is not an attempt to solicit business. This disclaimer is in addition to any disclaimers that this website has made. I am only licensed in California.
I suggest you go straight to the District Attorney's office. they will have a Assistant District Attorney that reviews your complaint and determines if there is a sufficient basis to press criminal charges or open an investigation.
You may wish to also report the incident to the school principal, the local school board, the state at http://www.tn.gov/education/district_info.shtml
You can also pursue a Civil claim for the injuries your son received.
But before you do anything you need to talk to a local attorney!
I hope your children are recovering quickly and completely-no child should have to experience this type of behavior.