Research on teacher stability typically focuses on the extent to which teachers remain in the same school, district, or the teaching profession from one year to the next. I investigate another facet of stability--whether teachers remain in the grade they teach. Drawing on administrative data from a large district in California, I find that high shares of teachers switch grades. Disproportionately, these are early career teachers who come from low-achieving or high-minority schools. Teachers who switch grades leave schools at higher rates than their colleagues and exhibit lower impacts on their students' achievement. For teachers who switch to a nonadjacent grade, these negative effects can wipe out any gains due to increased experience and can persist in the year after the switch occurs.