Teacher turnover has long plagued school leaders and administrators, particularly in urban districts. But a new CALDER paper suggests that teacher turnover may be even more serious than was previously thought.

For districts, turnover poses the challenge of finding replacement teachers that are at least as good or better than the ones who were lost. But what Matt Ronfeldt, Susanna Loeb, and James Wyckoff found is that even if a district can hire teachers of the same effectiveness as those who left, teacher turnover would still have a detrimental impact on learning. Teachers who remain in schools with high rates of attrition actually become less effective.

This conclusion may not come as a surprise. Nonetheless, it should inject a note of caution into the thinking of hard-charging school leaders who understandably want to move quickly to reform underachieving schools. And it makes it all the more imperative for districts to hire and support effective principals who can identify good teachers and encourage them to stay in their schools.

—Graham Drake