Rather than looking for the next big reform to improve teacher quality, a new study considers whether it's time for things to stay the same. Researcher David Blazar of the Center for Education Policy Research at Harvard recently looked into what happens when we play musical chairs with teachers' grade assignments.

For starters, teachers switching grades happens more often than you may think, ranging from about one in five teachers in any given year in some studies to more than one in three in others.

What Blazar finds is that this constant switching is not good for students. Using teachers' value-added estimates, Blazar compared the average returns to experience (the measure of how much a teacher improves due to more years in the classroom) for teachers who do and don't switch grades. Unfortunately, switching grades was almost never for the better.

For example, a teacher who switched grades between her second and third year of teaching generally reported 20 percent lower gains than teachers who had the same amount of experience but remained in the same grade. At some points in a teacher's career, the learning losses associated with switching grades lingered for at least two years.

The type of switch matters too. Those teachers who switched to an adjacent grade (e.g., 2^nd to 3^rd grade) generally fared better than those who made a nonadjacent grade switch (e.g., 2^nd grade to 4^th grade). This makes sense: a teacher faces a steeper learning curve, in terms of classroom management techniques, curriculum, etc., when she switches to a grade very different than the one taught before, compared to what she has to learn when teaching a similar grade.

Given that changing grades means that the teacher is likely to be less effective than she would have been had she stayed put, it's disconcerting that these grade changes were more common for less experienced and less effective teachers. Making matters worse, teachers in schools with low student achievement and higher proportions of low-income students and students of color also had higher rates of grade switching.

As Blazar makes clear, this all must be looked at in context. Grade assignments can be made for positive reasons. For example, it could be forced or chosen (a factor Blazar was unable to control for), and that intent could potentially make a difference in how well a teacher performs the following year. Nevertheless, there's enough evidence here to suggest that a principal should think twice about changes in teacher assignment.