Hanushek began studying teachers in the late 1960s. He was trying to understand what it is about schools that have an impact on how much students do — or don't — end up learning. He discovered that, of all the things that schools control, teachers matter most. More than class size, more than the curriculum, more than the amount of money spent per student. The best teachers get three times as much learning out of their students as the worst teachers, according to Hanushek's research.