In addition to all the great points about a child's readiness and ability to handle the double skip, I think a successful double skip relies heavily on the school environment. If the school is very diverse (for lack of a better word) in their tracking and class placement, then it will be socially easier on the child than if the school is very rigid. While the socially part might not be an issue in the lower elementary grades, I think there are some significant challenges with it in middle and high school.
