We had similar issues with DS when he was in second/third grades. Our approach was to supplement his math at home and at the same time just continue to push the elementary school. It took us almost two years but ultimately we were successful.

I think oftentimes the teachers are unable to tailor programs for children that are accelerated so in our case we purchased materials for him to work on at school. Not ideal as he had to work independently but we felt it was a better choice than having him learn nothing.