Earlier quoted context omitted.
Every single child is completely different, and has a different learning capacity and developmental time line. True, but most groups of 30-40 children are roughly the same. There will be a few smarties, a few morons, and a multitude of people in the middle. It's called the law of large numbers.
This presupposes that classrooms can be thought of as random samples of the child population. We know that they are actually geographically localized samples, so that presupposition is somewhat strained and needs a bit of defending, preferably by empirical means.
Or even better, you can compute a statistical predictor of his student's expected performance, and measure the deviation between predicted and actual performance.
The point is that individual variation in students is irrelevant. It is extremely unlikely that a teacher will get a class full of dumb kids.