I'm sure the solution depends on the kids--not all gifted/advanced kids are alike. For my kids, the elementary school offers a little bit of enrichment around whatever topic that the entire class does, with a bit more depth; the middle school offers acceleration (skipping grades). Neither works. The enrichment is not deep enough and the pace is not fast enough. The skipping of grades, well, the kids learn the stuff in the same shallow way that all kids learn, only earlier. We have known all along that my kids need to move at a much faster pace and the material needs to be much deeper. Our solution has been afterschooling and more recently online courses.
That is essentially the solution we are utilizing. We started this spring letting him take a online high school class at home. This has worked to an extent. We haven't told his elementary school as I'm sure it would be perceived as pushing versus giving a kid who craves more, more.