You cannot develop reasoning skills without first providing the tools. Grades 1-5 should focus on such tools. You also cannot teach according to skill in a one-size-fits-all system without separating the "gifted" students from friends and isolating them from the population at large.
We will not have the mixed classes of the one-teacher schools, with the older students helping the younger and giving them models to live up to, but we CAN put a fourth-grader with fifth-grade skills in a fifth-grade classroom for math. This sort of placement into classes according to ability would work even better as you get to the higher logic and abstract classes as the student achievement - per subject - determines placement. This is possible - except the administration would need to do more than appease the government and the teachers would need to give honest scores.