It's very very hard when people don't understand about giftedness. I know they're busy, I know they have a lot of students, I know, etc. etc. But all this doesn't mean you can just ignore someone.
Your child has a right to learn in school. This is what school is for. If the teacher can't see that, you might want to consider moving up the hierarchy.
Belle, is there anyone you can approach (eg the principal) and ask "Well, why is he here, anyway?"
Problem: he's been labeled as a behavior problem. Responses:
1. He's only six years old and isn't familiar with school and it's only the first week.
2. His educational needs aren't being met and this problem is creating the behavior problems. He is very unhappy right now and here's why....
3. His teacher seems to be inflexible and expects him to put in 100% of the effort needed to solve the problem. It would probably help a lot if some flexing came from the teacher too.
4. Suggestion: could he go to X grade for reading?
Remember this answer when they say "Everyone needs to learn to do things they don't like":
"I agree when the unpleasant task has a positive outcome, such as picking up toys or washing dishes. But doing tasks that are well below his capabilities has no positive learning outcome for him and is only a negative experience. He has a right to learn material at his level, and this issue is separate from behavior. How would you feel if you were forced, to the point of frustration, to do trivially easy mental tasks all day and denied an opportunity to be challenged?"
Belle, I'm so sorry. This is so awful for you and your family.
Val
