In a school setting, intelligence has two components: one is what you already know and the other is how fast you learn it. A proper intelligence based grouping

In a school setting, intelligence has two components: one is what you already know and the other is how fast you learn it. A proper intelligence based grouping has to account for both. If you have an algebra class with a 13 year old and an 18 year old the 13 obviously learns much faster and will blow away the 18 year old. Likewise, it makes no sense to put two fast learners in the same class if one is learning calculus and the other algebra. Interestingly enough, if you do it right it generally reduces to age groups further divided by learning speed. Making it more difficult... at younger ages 5-7 there is very little correlation between what you know and how fast you learn. My wife observed this as a kindergarten teacher. And making it even more difficult... sometimes something "clicks" and learning speed increases. Or you hit a brick wall and it decreases. The number of groupings increases rapidly and ultimately it becomes easier to lump everybody together and hope for the best. I'm not saying this is a good system. Just the most practical given the way schools are currently set up. A better way is a model where teachers act as rotating tutors giving students 1-1 help as they need it to move on to the next level.