It is. If you have a bunch of people tied together by chronological age, who only in extreme circumstances can be held back or advanced early, in a 20th century classroom environment all sitting together listening to one person "teach", then structurally, an environment is created that can handle only one speed of learning. There is no good choice for that one speed, because the standard deviation is too large. In the worst cases, since neither the teaching pace nor the learning pace is itself a constant, you end up with a situation that serves no one at all a good chunk of the time.
I'm not saying this was or is the deliberate intent; it's just what the structure inevitably produces. It's not clear how to structure it any other way with 20th century technology in a cost-effective manner (e.g., "classrooms of 3 people" was never on the table). I'm hoping we'll start cracking this nut in the next few years, though. The tech is increasingly there and it's the social inertia about what school has "always been" holding us back now.