Schools and friendships represent important but distinct contexts for adolescent identity development. However, research has yet to explore the long-term interplay between these factors on ethnic/racial identity (ERI). This study included a sample of 640 adolescents from 9 public high schools in a diverse United States metropolis (Mage = 14.50, SD = 0.67; 44% Asian, 20% Black, 36% Latinx; female = 68%, male = 32%, non-binary = 0%). Latent growth curve modeling was conducted to investigate longitudinal associations in friendship ethnic/racial composition and ERI exploration. From the 9th-11th grades, same-race friends and ERI exploration increased linearly whereas friendship ethnic/racial diversity decreased linearly. Adolescents attending more ethnically/racially diverse schools maintained more ethnically/racially diverse friends over time but did not differ in changes in ERI exploration compared to adolescents in less diverse schools. There was no association between the rates at which adolescents' friendship ethnic/racial composition and ERI changed over time. More ethnically/racially diverse friends in the 9th-grade predicted faster increases in subsequent ERI exploration. The findings highlight important differences in the roles of friendship and school contexts on ERI, suggesting that friendship ethnic/racial diversity, but not school ethnic/racial diversity, facilitated ERI exploration over time. School ethnic/racial diversity did facilitate a slower decline in friendship ethnic/racial diversity, emphasizing the importance of school integration.