Acculturative stress has demonstrated significant negative relationships with Latinx students’ academic outcomes. Framed through interpersonal contact theory, we examined the moderating roles of school ethnic representation (proportion of same-ethnic peers) and school ethnic diversity (proportion of different ethnic groups in student body) in the relationship between acculturative stress and both academic self-efficacy and school belonging. A sample of 190 Latinx youth (14–18 years old) completed measures of acculturative stress and school functioning, and they provided the name and location of their high school to facilitate calculation of school ethnic composition. Both ethnic representation and ethnic diversity moderated relationships between English Competency Pressure (ECP), a specific form of acculturative stress, and academic-self efficacy. School ethnic diversity also interacted with ECP to predict school belonging. Negative relationships between ECP and academic self-efficacy or school belonging only emerged for students in schools with very low ethnic diversity. In contrast, ECP was significantly negatively related to academic self-efficacy only for students in schools with the highest ethnic representation. These findings add complexity to the discussion of how school context matters for Latinx students’ academic performance.