I've accepted that for my kids, I have to teach them at home. End of story.
This is what I would have posted too. And another excuse that I have heard in addition to all the above: We only teach to the top 80 percentile of the class and if your child is in that range, his needs will be met. Ha!
Challenging curriculum in lower elementary math for my child has been the jump from, 4+5 to 14+15 kind of stuff. But, he is generally happy because he is with other kids, he gets to do new stuff in math - they let him measure temperature with real thermometers, count pennies and work on geoboards all of which he loves to do.
My child is drastically accelerated - he is taught outside of school. School is for fun! I am not trying to put a damper on your enthusiasm. If you can make it work, that would be the best thing for your child. If not, don't give up hope. Explore how you can teach or get your child into a local enrichment club or afterschool program that meets your child's intellectual needs.
Good luck.