I am of the opinion that the obvious answer is, "Teach each to his/her ability", but is that something that is doable?
It is doable with small classes. My son's small private school teaches each child at his/her level of ability. Students are pretested at the beginning of the year and when new material is introduced. Then, the teacher plans and designs an individualized curriculum for each student based on the pretest results. Although the kids stay in the same class as their agemates, each child has different assignments and goals. In my son's kindergarten, some kids got kindergarten level work. Some got 2nd grade math and 1st grade reading. Some got 1st grade math and 4th grade reading. But there were also less than 10 kids in the class, with one teacher and a teacher's helper. The entire school, which runs from K-12, has less than 100 kids total.
