If there really were a single dimension axis of smartness, won’t there be a “smartest” and a “least smart” in every classroom? And if they’re in the wrong class and they leave, won’t there be two new pupils at the extremes? This argument of “you’re in the wrong class” always sounded elitist to me.

The important is that the teacher tailors the teaching to the students. Spend more time on the ones who struggle, give extra stuff to do to the quickest (e.g. help teaching to other pupils).

I’ve also always been against separating children by “intelligence”. Having a “smart” class and a “dumb” class is a self-fulfilling prophecy.

That being said, there are children who have special needs and who require a teacher who has the proper formation to help them.