I don't think your being unreasonable, in all honesty I think the issue lies with what she expects from him and the way she goes about it.

Sending snarly notes home is not going to help things. Pushing you to have your child assessed is not going to change things.

It doesn't sound like she is very "adaptable".

When I was having issues with DS1's teacher, we organised for a meeting after school. DH and I set out to her what our expectations were, both of our son, and what we expected from her, as the teacher. We then set some mutual goals for DS1 to achieve and had a genersl conversation about the curriculum, what's expected of that grade level student etc. Since the meeting DS1 has been a different kid at school, and the teacher has adapted to meet his needs as well.

I highly recommend sitting down and having a chat with the teacher as a starting point.

Good luck