Actually, the argument for skill-based education _is_ that learning speeds are different. So for one it takes 6 months to learn something, for another - 3 year

Actually, the argument for skill-based education _is_ that learning speeds are different. So for one it takes 6 months to learn something, for another - 3 years. And in such systems it's fine - you got promoted because you have learnt something, not just because you got one year older (and haven't failed utterly). And again, for the same "level" of subject there can be two speeds - so no risk of smart 8 yo kids in the same class with slow 11 yo.