Matt Yglesias’ five-year old son asks: why do we send the top students to the best colleges? Why not send the weakest students to the best colleges, since they need the most help? This is one of those questions that’s so naive it loops back and becomes interesting again.
To avoid corrupting the youth, we might provide an optimistic answer: anyone can teach addition, any college math major can teach calculus, but it takes a world expert to teach ten-dimensional hypertopology. We want to take the few students smart enough to learn hypertopology and connect them to the few experts smart enough to teach it. But this seems false; most of the classes at top colleges are the same material that gets taught everywhere else; you don’t get into subjects that need world experts until postgrad.
Another answer, still somewhat optimistic: we want to maximize the chance of geniuses doing revolutionary work. If we give a mediocre student the world’s best writing teacher, and a genius a mediocre writing teacher, they might each write a pretty good novel. But if we give a mediocre student a mediocre writing teacher, and the genius the world’s best writing teacher, then we might get one mediocre novel and one work of staggering genius which revolutionizes literature forever. Likewise, if we connect the world’s most talented young scientists to the world’s best science teachers and labs, maybe they’ll cross some threshold of understanding where they can discover a cure for cancer. I think this is the best explanation that sticks to optimistic prosocial answers.