<p>My theory is that the teachers who are really bad end up teaching the weakest students, because their parents are less likely to complain about poor teaching. The better teachers and the parents of higher-ability kids want the same thing, and they usually get it. The parents of the weak students may defend their kids from disciplinary claims, but they’re not going to complain about pedagogy.</p>
<p>If you force the best teachers to teach the weakest students, and have weaker teachers teaching the best students, the good teachers and the good students will bail for private school.</p>