Recent attempts to measure schools’ influence on students' SEL show differences across schools, but whether these differences measure the true effect of schools is unclear. We examine the stability of school-by-grade effects on students' SEL across two years using a large-scale survey. Correlations among effects in the same grades across different years are positive but lower than those for math and English. Schools in the top or bottom of the effect distribution have more persistent impacts across years than those in the middle. Overall, the results suggest that these school effects measure real contributions to students' SEL. However, their low stability draws into question whether including school value-added measures of self-reported SEL in school performance systems would be beneficial.