Grade retention ('holding kids back') has additionally dropped significantly since the average HNer has gone to school. I remember going to school where one of my peers went to sixth grade with his brother two years older than him. But now, we give out social promotions.

That might've worked if we funded schools & gave students who fell behind significant interventions & 1x1 attention, but that's not what happened. One of my friends has a very bright and talented fifth grader in a class with multiple students who can barely read or write. Guess who gets the most attention from educators? Which group the teachers structure the class for?