I once yelled at a doctor because after a week of treatment for a specific problem, my father wasn't getting better. I was sure Dad didn't have problem A, but that he had either problem B or C. A week of polite, patient questioning and suggesting things to the doctor had done nothing to change the doctor's mind.
The morning after my "temper tantrum," there was a parade of different medical personal in and out of Dad's hospital room. Two confirmed that Dad did not have problem A. A few others figured out that he did indeed have problem B. After 24 hours of the correct treatment, Dad was 75% improved. He was discharged from the hospital in five days.
My brother has a child with physical disabilities who requires a one-on-one aide in school. One year, Nephew had a toxic aide. My brother and SIL tried to work with the school for five months, attempting to get the aide the proper training to work with Nephew, or at least to stop her from making things worse--she prevented him from doing the few independent things he could do, kept telling him that he was crippled and would end up in a home or worse, that sort of thing. Other adults in the classroom verified this. I noticed a huge change in my nephew after two months of this aide. He was depressed, sad, scared, instead of the bright, happy, active, alert child that he had always been. He never cried before, he was always crying. He never mentioned his disabilities before, he was always crying about them.
It wasn't until my brother stormed in to the principal's office and demanded that a new aide be found immediately and until that happened, he (my brother) would be attending school daily and acting as his son's aide that the toxic aide was removed. A new, wonderful aide was found in two days.
I don't advocate that people throw temper tantrums and issue ultimatums. But I think that sometimes politeness is seen as agreement, or that you aren't going to push the matter further. And sometimes, with some people, you have to get loud/angry/mad to be heard and taken seriously.
I'm not exactly proud that I yelled at that doctor. I am happy that I managed finally to get Dad the medical treatment he needed. If I had known a better way of getting that done, I would have used it.
The OP should be glad that her son has a good teacher now. The school and the principal should be apologizing to her and her son for giving him a completely inadequate teacher.