The Irreplaceables

This study by The New Teacher Project, which looks at 1.4 million students across four urban school districts, directs its focus on the experiences of some of the highest performing teachers. That is, teachers who have had so much success in boosting student learning that they are essentially “irreplaceable.” The study found that the most successful and unsuccessful teachers exit urban schools at similar rates, as schools rarely make a strong effort to keep the best ones. This leads to about 10,000 “irreplaceables” in the 50 largest districts leaving each year. 
The New Teacher Project