...if you are spending more time, energy and resources trying to advocate for your child and not getting any cooperation you're better off putting that time, energy and resources into homeschooling.
Yeah, that. Or finding another school.
My eldest is homeschooled after a very rough year last year. Now he's happy and learning and doing well. But homeschooling isn't for everyone for a variety of reasons. If it isn't an option for you, can you move your child to a new school?
I think you've had some good advice here. I especially like the advice to figure out what the law is (an attorney can do that for you, of course) and to write stuff down. Written records are very important. A person can say, "No, that's not what I said," but it's hard to say, "No, that's not what I wrote."
Good luck.
