When I was in primary school, it was pretty evident that I was bored in class, simply because it was too basic. You know what they did? The just pushed me forward a year. And then another, and another, and another. This meant I was 10 when I started high school. You know what sucks about being 10 in high school? Everything. Other kids are assholes - even more than usual - because you make them look bad. Teachers expect more from you, but at the same time, they don't really want to put up with you. Even PE is bullshit at that point, because 10-year-olds suck at physically keeping up with 14-year-olds.
I'm not sure about the numbers, so I don't know if this is a worthwhile endeavor, but here's what I always thought would be a better solution: gifted students should progress at a social pace similar to other kids. This means they would be in a class with other students their age who had also been placed in the gifted student program up until the age of 17 or 18, when they would normally graduate high school anyway. The major difference would be that these students, at a time deemed fit by qualified educators, would begin earning college credits. That way, they would have a running start upon entering college, and not be socially crippled.