Jool, I'd say relax. You are doing the right thing, but it doesn't mean that it will work out everytime you try. Of course we parents all want to make sure that our kids are in an ideal environment, but kids are resilient, if once in a while things don't work out exactly the way we like, chances are kids and parents will be able to cope.
I say this out of my own experience. My DS8 had a terrible year in last year. He was bored silly during math--and he had no weak areas whatsoever--yet the teacher refused to give him any challenge. We talked with the teacher a few times. The answer that we got was "well I know your son is smart but there are many smart kids in my room, some smarter than your son, and none of THEM is bored with my teaching". We didn't fight, because for various reasons we weren't ready to switch DS to a different room or a different school in case that our relationship with the teacher went completely sour. Instead we helped him find interesting material to work on at home, and we made sure that he had enough intellectual stimulation with his extracurricular activities.
This year the new teacher immediately saw that DS's math ability is way ahead of other kids in the classroom, and she gave DS advanced material from the very beginning of the school year and DS works on it at his own pace--as soon as he finishes one level he gets the next level. DS's enthusiasm for math is completely restored.
So what I'm saying is that we should definitely try to arrange the ideal situation for our kids. But we should not put too much pressure on ourselves. If for part of the time in school the kid is not fully challenged it's probably OK. With different teachers each year, things will go better or worse. We can use other means to satisfy our kids' need when things at school don't go so well. So, enjoy the challenge!