Typically kids are learning a series of loosely related concepts that require different skillsets to process. If for one person integrals are intuitive but vectors are not, while for another vectors are intuitive while integrals are not, both would average roughly the same rate of learning, but you are still faced with the same dilemma - either you're slowing down to make sure the one student gets the concept they struggle with at the expense of the one who wasn't struggling or you move ahead quickly at the expense of the straggler. Both students would appear to be equal performers but that doesn't mean it made sense to group them together. While perhaps classes could be designed to have good correlation between concepts, you certainly couldn't do so for a well rounded education - there is absolutely no reason to believe someone who learns mathematical concepts quickly would also pick up literary concepts quickly. There may be small populations who do learn everything quickly and others that learn everything slowly, but certainly the overwhelming majority of the population would fall in-between.