This short-term longitudinal study investigated 918 students' school-related affect across the transition to high school. The study focused specifically on the moderating effect of change in student ethnic congruence from middle to high school. Results indicate that students experiencing more ethnic incongruence from middle to high school, in particular African American and male students, reported declining feelings of school belonging over time. Moreover, students experiencing ethnic incongruence also had increasing worries about their academic success. These results suggest that the changing school demographics from middle school to high school may negatively impact students' school-related affect, especially if they move to high schools that include fewer students who are ethnically similar to themselves.