Highered Intelligence looks at the question I raised on skipping, and finds the alternatives -- tracking or homeschooling -- just as unpalatable. Money quote:

This issue comes up, of course, because we have decided as a nation to meld academic and social development. We have made specific decisions that cause our children to identify with their class. Having a birthday party? Invite all the children in your class. Making cookies for Christmas? Bring them for everyone in your class.

The classroom has become, perhaps by design, become something on par with family in terms of identity formation.

It is precisely because we have joined these two processes that accelerating a child academically presents difficulty for the child socially. We as a society do not have the tools necessary to separate these goals (social and intellectual development) on an institutional level.