This study used a natural experiment design to examine the impact of ethnic studies courses on students' ethnic-racial identity (ERI) development, multicultural attitudes, and civic engagement during the 2021-2022 school year in Minneapolis, MN (N = 535; 33.5% White, 29.5% Black, 21.1% Latine, 10.7% multi-racial; 44.7% female, 7.1% non-binary). Compared to students who were quasi-randomly assigned to a control class, 9th graders taking an ethnic studies class (treatment group) engaged in significantly more midpoint ERI exploration ([beta] = 0.12), resulting in stronger endpoint ERI resolution ([beta] = 0.48-0.57). Increased exploration mediated more favorable attitudes toward multiculturalism (indirect effect = 0.05) and more frequent civic engagement activities (indirect effect = 0.02). Results have implications for policy efforts to expand ethnic studies.