When teachers leave schools, overall morale appears to suffer enough that student achievement declines—both for those taught by the departed teachers and by students whose teachers stayed put, concludes a study recently presented at a conference held by the Center for Longitudinal Data in Education Research.

The impact of teacher turnover is one of the teacher-quality topics that's been hard for researchers to get their arms around. The phenomenon of high rates of teacher turnover has certainly been proven to occur in high-poverty schools more than low-poverty ones. The eminently logical assumption has been that such turnover harms student achievement.

But a couple years back, two researchers did an analysis that showed, counter-intuitively, it's actually the less- effective teachers, rather than the more- effective ones, who tend to leave schools with a high concentration of low-achieving, minority students. It raised the question of whether a degree of turnover might be beneficial, since it seemed to purge schools of underperforming teachers.

When reporting on that study, I played devil's advocate by pointing out that it didn't address the cultural impact of having a staff that's always in flux. The recently released CALDER paper suggests I may have been right in probing this question.

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