The Economist argues that schools tend to broadly fall into two equilibria - the "good" and "bad" ones. These equilibria are stable owing to virtuous/vicious cycles involving good students. So while the good schools attract good students who in turn attract more of their ilk, the reverse happens in bad schools. The trickiest part is to determine the percentage of good students required to push a school from the bad to good equilibrium and vice versa. This model however seems to neglect the effect of good teachers who can act as alchemists in transforming a mediocre student to a good one.