I think I'd focus on the communication issues between the teacher and you first. That's an overarching problem that could be extremely damaging in the long run. Second I'd focus on her issues with your child and the unfairness of being judged to be lacking for a diagnosed issue.
Unless she's failing him, I think I'd do as much as I could to minimize the focus on grades. The kid is in 3rd grade. So what if he gets a B? That's not really the heart of the issue. The real problems are her unclear communication style and her unfairness to a disabled child.
