Bianca's experience is the polar opposite from mine. Our public school did nothing for kids ahead of the curriculum in second grade, so the more DS went ahead, the more problematic daily school became. As I recall from Bianca's previous posts, her DD's daughter's school is fabulous and attentive to her daughter's rapid progress and accelerates in the classroom accordingly. (True? I love reading about your DD's school and her phenomenal success there!) I think ebeth raises a good point - the farther afield of the curriculum your child gets, the bigger the problem without in school accommodation. But no reason to hold a child back arbitrarily. If I knew more, I could have found activities that taught more depth rather than accelerated subjects, I suppose. But I also didn't want him to sit there for a year without learning anything new or doing challenging work. I certainly don't have the answer, but I think it's helpful to see both extremes in back to back posts.
