Perhaps the most pervasive feature of schooling around the world is that students are grouped according to age — with 8-year-olds in one class, 9-year-olds in the grade above, and so on. The idea is that pupils of the same age will be at approximately the same level and can therefore benefit from the same instruction. This assumption is often wildly wrong. In many classes, especially in the poorer part of the world, the educational ability of children in the same grade varies widely.
