I think it's possible in some schools, for some kids... Probably if you're in a large district with a lot of resources you could add on more and more specialty classes, beef up the arts program, do some independent study and internships... I don't see that happening in our local PS system (they have a full complement of AP offerings and all that, but nothing fabulously interesting that would keep one around for any longer than necessary...)
The more flexible they're willing to be the better, I think. And it probably helps to have a kid who can work independently or move between groups with ease... since it's unlikely you'd get a class-sized bunch in one place at one time.
You'll never actually "run out" of things to learn, and there are some areas that the traditional class sequence leaves out, which I personally thing are absolutely vital to all reponsible citizens (not just the gifted ones!) like more understanding of law and governance and international relations, a better grasp of statistics and science, more foreign language... Since we have "extra" time, we're spending a good bit of it on all of those!
