Ethnically and racially diverse schools provide students opportunities to socially interact with both same- and cross-ethnic peers that can shape their sense of belonging within a school. This study investigates the extent to which same- or cross-ethnic friends influence feelings of school belonging in two large, diverse U.S. high schools (total N = 4,461; 9th-12th grade; 49.6% girls). Employing a longitudinal social network analytic approach, using stochastic actor-oriented modeling, this study found that students become more similar or stay similar to their same-ethnic friends, but not cross-ethnic friends, with no clear indication that students select friends based on similar levels of belonging. These novel findings highlight how feelings of school belonging are fostered through sociability in same-ethnic friend groups. Implications for interventions and other approaches to enhance school belonging are discussed.