That is why the for-profit Asian tutoring industry groups students by performance, not by age. There are “grades,” but they do not depend on when a student was born, only on what she knows and is able to do.
I just interviewed Andrew Hsu for Startups Open Sourced and he mentioned this was very important in education. He had scored so high on his IQ test at 6 years old he was classified as "genius" and received 3 B.S. degrees at 16, and then dropped out of his Stanford Ph.D. at 19 to do a startup. One thing he says really makes a difference is splitting students based on skill level, not by age. Hoping to release the interview soon.