For what it's worth (all children are different) my ds had a miserable year last year. We worked hard on helping him to learn ways of coping with a teacher that really was pretty sub-standard and a little odd--(his instincts were right on!) We never let on to him that we thought she was a bad teacher, even though we did, but helped him figure out ways to supplement his learning at home, deal with her peculiarities, etc. It felt very hard for all of us at the time.
Now, this year, he is so appreciative of his new teacher, whom he loves! I think he feels all the more so from experiencing a bad teacher last year. And I think he has learned some lessons about putting up with things you might not like and figuring out how to get something out of it anyway. We did have to have words with the teacher a couple of times when she got out of line, but we tried to keep this separate from him, so he didn't see us as trying to 'bail him out.' This is what worked for us, and I have no idea if it will for you. A school year can feel very long when our children are unhappy!!