If there really were a single dimension axis of smartness, won’t there be a “smartest” and a “least smart” in every classroom?

Moment to moment, presumably. But your cognitive ability waxes and wanes for a host of reasons - mood, exhaustion, calorie consumption, experience on the problem set. If you test ten kids over ten iterations, and each testing gives you a different permutation of rank, which kid is the smartest? If you have four kinds of intelligence exams and four different kids all place 1st in one of them, who is the smartest? Is the kid who aces Numeric Problems but flunks Word Problems smarter or dumber than the kid who middles in both?

The important is that the teacher tailors the teaching to the students.

Sure, which is why you want to cluster kids by current ability rather than some holistic but ambiguous attribute like IQ score or head shape. But you don’t really see this sorting by ability until upper-end high school elective classes (sorting the Bio 1 kids from the Bio 2 kids or the Honors musicians from the fuck-offs).

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

Sure sure. But we’re defunding all that under the current administration, so its a moot point.