For a long time, we've heard about the damage done by teacher turnover. Often, the thinking is that schools struggle to replace the teachers who leave with replacements who perform at least as well or better.

And while it is true that high turnover can really hurt a school, it turns out that the damage is largely caused by the ripple effect of a teacher leaving, mainly the amount of grade switching that occurs. A new working paper from Eric Hanushek, Steven Rivkin, and Jeffrey Schiman measures the broader impact of teacher turnover in a large school district in Texas.

When a teacher leaves a school, the remaining teachers often have the opportunity to move around, selecting what might be considered 'better' grades and leaving the more 'difficult' grades to be taught by newer teachers. Not only do the new teachers have to deal with the more difficult assignment, but the teachers who switch grades, as some other recent research has found, are also less effective, at least in their first year teaching the new grade.

Not as new but always worth restating:  Hanushek et al. again find that the weaker teachers were more likely to leave their schools than strong teachers, which means that unfocused efforts to reduce turnover are not necessarily in the best interest of schools.