The education research of recent years has pointed overwhelmingly to the importance of teachers. Perhaps more than anything else – quality of principal, size of school, size of class – the strength or weakness of classroom teachers influences how much students learn and even how they fare later in life. The great unknown is how to improve teacher quality, be it by attracting more good teachers, weeding out more bad teachers or helping teachers become better at their craft. A new study, released on Thursday, offers powerful if still tentative evidence that teacher-evaluation program...