DS was like this and certainly could have finished grade school curriculum very quickly. But he liked being in grade school. He enjoys kids his age more than older kids; he likes the activity; he gets on well with the teachers. He has a lot to learn from the "extras": music, PE, art. His teachers have been good about giving him more challenging work when they are able and he is OK with waiting (and entertaining himself in various ways) when the teacher is working at the other kids' pace.
We do lots of fun afterschooling and I feel comfortable that he is learning. One of the things about his not being challenged at school is that he doesn't have much homework and so there is more time to supplement at home.
I think a lot depends on the kid and how they fit with the school they are in. DS has always been enthusiastic about going to school so that has made our decision clear.
Obviously a less enthusiastic kid would require a different approach!
