Schools in the United States are increasingly offering ethnic studies classes, which focus on exploring students' ethnic-racial identities (ERI) and critical analysis of systemic racism, to their diverse student bodies, yet scant research exists on their effectiveness for students of different ethnic-racial backgrounds in multiracial classrooms. A policy change to require all high school students in one school district to take an ethnic studies class facilitated a natural experiment for comparing the effects of quasi-random assignment to an ethnic studies class (treatment) relative to a traditional social studies class (control; e.g., U.S. Government, Human Geography). Student surveys and school administrative data were used to compare students' ERI development, well-being, and academic outcomes across ethnic studies and control classes. Participants (N = 535 9th graders; 66.1% ethnic studies) had diverse ethnic-racial (33.5% non-Latine White, 29.5% Black, 21.1% Latine, 10.7% biracial, 2.8% Asian, 2.2% Native American) and gender identities (44.7% female, 7.1% non-binary). Ethnic studies students reported marginally higher ERI exploration and resolution than controls, and sensitivity analyses showed a statistically significant effect on ERI among participants with complete midpoint surveys. Higher resolution was associated with better psychological well-being for all students and higher attendance for White students. Students with low middle school grades (GPA < 2.0) had better high school grades in core subjects when enrolled in ethnic studies than the control class. Overall, the results of this natural experiment provide preliminary support for ethnic studies classes as a method for promoting ERI development, well-being, attendance, and academic achievement for students from diverse ethnic-racial backgrounds.