Imagine that you are a little behind in soccer ability. You never played soccer in high school or college, you are not the most athletic person. Now in your 20s you want to sign up for a soccer league. The local league has multiple divisions for different ability levels. What do you do? Do you sign up for the highest, so that you can be exposed to great play? Or do you sign up for the division that matches your own ability? The answer for most people is obvious. You sign up for the division that matches your own ability. If you sign up for a top-level league, no one will pass the ball, and when you get the ball, you'll have it taken away so fast you won't have a chance to get better.
The idea that the best way to help someone who is behind, is to put them with those farthest ahead, is completely insane. Yet that has been the policy behind affirmative action, behind integration-via-fiat, and behind eliminating tracking in grade school. It is not helpful to be in classes with people who are far ahead of you, as you won't get the practice on the basic, foundational topics that you need. You will be lost, and then fall even further behind. On the other hand, if you are grouped with people of your own ability, you have the opportunity to work hard, be among the best in the class, and feel much greater personal satisfaction. (See again how many nerds who despised gym class end up later liking casual ultimate frisbee games -- such games among people of similar ability level ends up being a lot more fun).
