My DS started to show this issue in first grade. We moved him to a self paced environment because he was already checking out (at six years old, it can happen).
It's a vicious cycle- the more he underachieves, the less the teachers are willing to accelerate, the more he underachieves, etc. etc.
indigo has some good ideas. I don't have much to add, except to say that we were not successful in advocating for advancement in a traditional school.
One thing that has helped DS (he's in second) is that his new school offers a lot of depth. For instance, they use Singapore math-- rather than rapid acceleration (DS is working almost two years ahead now), once DS completed the general workbook, they moved him on to extremely rigorous problems using the learned methods. I'm mentioning this because there are some good curriculums/tools that could be used for enrichment-- either at school or at home.
