From Teacher Beat by Stephen Sawchuk:
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.

Among their findings:

• For each analysis, students taught by teachers in the same grade-level team in the same school did worse in years where turnover rates were higher, compared with years in which there was less teacher turnover.
• An increase in teacher turnover by 1 standard deviation corresponded with a decrease in math achievement of 2 percent of a standard deviation; students in grade levels with 100 percent turnover were especially affected, with lower test scores by anywhere from 6 percent to 10 percent of a standard deviation based on the content area.
• The effects were seen in both large and small schools, new and old ones.
• The negative effect of turnover on student achievement was larger in schools with more low-achieving and black students.

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