Students spend almost all of their time in schools under the direction of a teacher, so in debates about the most important school inputs, it comes as little surprise that many argue teachers are the most important input. Recent research measuring teacher performance with value-added measures has helped quantify teacher importance by showing the vast differences in performance between the most and least effective teachers. For a single year, having one of the best teachers is associated with an 8 percentile point increase in achievement over what it would have been with one of the least effective teachers, and these differences may add up over time if there is a sequence of high value-added teachers.

This evidence about teacher performance has struck a chord with policymakers, ...