This is not the case in Japan. Children's needs are met by engaging their thinking. Otherwise, low achieving students miss the opportunity of having students, other than ones with similar learning difficulties, model for them alternate ways to solve problems. High achieving students in mixed ability groupings have to find ways of interacting, and communicating ideas, and this really benefits them. This is not to say there is no tracking or setting of students.
Asian countries delay tracking.
In Japan, up until 10th grade, classes are heterogeneous. This allows for different rates of the developmental trajectory. We see it all the time, children who don't get it one day and then suddenly they take off. We over emphasise academic intelligence, we have a one dimensional view of gifted and talented. The Japanese system allows for individual differences, different types of achievement - leadership, communication skills etc.