Well can't send my son back to THAT school next year

My son has been budding head with his English teacher all year. We moved and he changed schools. He was in advanced classes at his old school, but credits got messed up in the transfer and he was put into normal classes. He was constantly getting sent to the principals office for calling out whenever his English teacher was teaching him something wrong. To be fair I have had people tell me this isn't a good school and their test scores are really low. He had even showed her in their English book to prove she was teaching it wrong. So she would just send him to the principals office. My friend who is a college English teacher went over his notes that his teacher had him write and even she was pointing out things that were wrong. So Monday he got his final essay back and he comes home in a panic attack. He got a C. He's never gotten a C before. He is panicking and saying he doesn't know what was wrong with it. My friend came over and went over his essay. The teacher had circled where he used "improper grammar" and wrote on the side what was wrong with it. She handed me his essay (I am not good at English) and said "Go to the school." She said most of what she said was wrong with it and was improper isn't and if she was heading it as a college teacher that would at the very least be a B essay. So she came with us to the school. Principal called his teacher in and it was a huge argument. The principal brought in the remedial English teacher to get a second opinion. She goes over it and starts saying "No he used that correctly." "No that's not improper." She was showing his teacher a sentence and said "No a period does need to go there. That can't be one sentence or else it's a run on sentence. He did that right." These remedial English teacher is calling this teacher out on all the things she said that was wrong when it actually wasn't. The part that made his teacher turn scarlet is when the remedial English teacher said "Umm he didn't use the wrong there... T-H-E-I-R shows ownership... Not T-H-E-Y-R-E." Finally she said "Fine I'll change his grade!" Well that was settled. I'm gonna have to find him a new school because that one left a bad taste on his mouth, he's too advanced for it, and I'm pretty sure he made some enemies this year.