According to a new study by The New Teacher Project, U.S. school districts need to improve their retention strategies because the strongest teachers are just as likely as weak teachers are to leave their schools after five years. The study looked at four urban districts, including New York City. Its authors say they relied on student test scores to identify the top 20 percent of teachers, whom they call the “irreplaceables”–teachers whose students consistently make the most progress on state exams, year after year. On average, they help students learn two to three additional months’ worth of math and reading compared with an average teacher, and they get high marks from their students. The report recommends teacher merit pay and better teacher evaluation systems, two hot-button issues contested by the unions representing teachers and principals. The report also says school districts should evaluate principals partly based on how well they do in retaining the best teachers.