What axis do you pick? Kids can be brilliant in one subject but terrible in another. They can have behavioral problems but vary between low and high achievers

What axis do you pick? Kids can be brilliant in one subject but terrible in another. They can have behavioral problems but vary between low and high achievers. The reality of it is that we group kids by age and geolocation and put them under the same teacher. It works out mostly ok, improving that will be a hard problem. Even defining an improvement is not trivial as well. While you ask if students can be grouped by ability, some schools prefer to split these students up, with the thinking that there is more to be gained by having children helping each other out in the same room.