While much empirical research concerns job tenure, this paper introduces the concept of school tenure and further examines whether and how school tenure impacts student outcomes using a rich set of cohort data from England's secondary schools. Using the number of times a student switched schools during the academic year as an instrument to measure school tenure to fully account for endogeneity issues, the resulting two-stage least squares estimates suggest that the effects of school tenure are positive and heterogeneous across students. While advantaged students are more likely to gain when their own school tenure is longer, disadvantaged students benefit if their peers have longer tenure. This significant heterogeneous effect suggests that school tenure may not simply act as the inverse of school mobility but may represent a potential determinant of student achievement and an effective school-specific measure for policymakers and practitioners. The relevant implications are accordingly discussed.