In-class differentiation is a false utopian myth, in most cases. It is far more effective to group school children by ability in separate classrooms, and teach to each group's level.
"Differentiated instruction" has been promoted so strongly that it may be best to pay it lip service. Even if you divide 60 children in a grade into three ability groups for reading and math, there will still be variations in ability and background within the 20-student classes. So let teachers "differentiate" as best they can, but make their jobs by reducing the range they need to deal with. After all, even believers in differentiation don't take 300 children in grades 1-5 and assign each teacher 4 children in each grade (for a total of 4*5 = 20 per class).
In short, ability grouping and differentiated instruction can be viewed as complementary.
