There is no shortage of research on the importance of good teaching. For decades, study after study has shown that there are large differences in effectiveness from one teacher to another and that these differences can have a lifelong impact on students. A recent study that tracked 2.5 million students over 20 years determined that those with…

"Irreplaceables" are teachers who are so successful they are nearly impossible to replace, but who too often vanish from schools as the result of neglect and inattention. To identify and better understand the experience of these teachers, the authors started by studying 90,000 teachers across four large, geographically diverse urban school…