As a teacher, I agree that your difficult child's math teacher was totally out of line. You need to write a complaint to the superintendent and principal NOW, so that bad eggs like her don't ruin the entire profession. I am so sorry for what your son is going through, but I understand it as my difficult child had a similar situation. Unfortunately, it was parents who ganged up on my son and the teacher, and since it was at a private school, my son was not allowed to re-enroll for the following school year. I wouldn't have allowed him to go back anyway, especially after the way we were treated.
When I requested an IEP, I hand delivered the letter to the principal and got a written receipt, and sent a copy to the Asst. Sup. for Spec. Ed via certified, return receipt mail. Because the school year is almost out, there is little chance that your difficult child will be tested soon. In the meantime, you may want to get private testing that you can provide to the school district in the fall. Many school districts don't accept it and will want to do their own testing, but if there are any discrepancies between the private testing and the school district's, it gives you some leverage for either retesting or at least an appeal.
Definitely take a look at the Special Education Forum for more information. Good luck!