This article examines the extent to which normative beliefs on acculturation constitute (a) individual resources and risk factors for adolescents facing developmental tasks and (b) institutional norms that define developmental milieus in secondary schools. To what extent do egalitarianism, multiculturalism, assimilationism, and segregationism help or hinder academic and psychosocial adjustment in mid-adolescence? We examined how both individually endorsed beliefs and socially shared beliefs at the 10th-grade cohort level relate to students' academic performance, educational and occupational aspirations, motivational and emotional attachment to school, self-esteem, and life satisfaction. Data came from a random sample of 15- to 17-year-olds in Berlin, Germany, assessed at the end of Grades 9 and 10 (N = 1,992). Multivariate analyses identified egalitarianism and multiculturalism as the most prevalent acculturation beliefs, forming a combined belief pattern that pairs recognition of equality with respect for diversity. At the individual level, this belief pattern was positively related to academic and psychosocial adjustment. Assimilationism beliefs were associated with higher motivational and emotional attachment to school in both minority and majority students. Segregationism beliefs were associated with higher self-esteem in all adolescents regardless of background, but with lower levels of academic performance in immigrant youth. At the grade cohort level, a single bipolar factor (egalitarianism/multiculturalism vs. assimilationism/segregationism) described the institutional diversity culture. An environment characterized by egalitarianism/multiculturalism beliefs was positively associated with motivational and emotional attachment to school but not related to academic adjustment or psychological well-being.