Eric Hanushek, an economist from Stanford, estimates that the students of a very bad teacher learn, on average, half a year’s worth of material in one school year. The students in the class of a very good teacher will learn a year and a half ’s worth of material. That difference amounts to a year’s worth of learning in a single year. . . . After years of worrying about issues like school funding levels, class size and curriculum design, many reformers have come to the conclusion that nothing matters more than finding people with the potential to be great teachers. But there’s a hitch: no one knows what a person with the potential to be a great teacher looks like.