Schools and communities are increasingly diverse. Therefore, it is critical to understand diversity’s impact in school, including how diversity relates to students’ psychosocial experiences and who is thriving or experiencing more challenges in a high diversity context, to inform practices that support students in culturally diverse educational environments. This dissertation assessed school diversity’s relations with students’ psychosocial experiences built upon cultural-ecological theory using a sample of California public school students. Considering the limited diversity measures, which show inadequacy in measuring dual-concept diversity in educational research, this dissertation also discussed diversity measures and proposed alternative mathematical formulas to capture diversity. Three studies were structured. Study 1 used latent profile analysis to explore school diversity profiles indicated by student racial/ethnic and socioeconomic diversity and teacher racial/ethnic diversity among California public schools. Multilevel multigroup analysis was employed to assess profiles’ relations with two indicators of psychosocial experiences (i.e., race-based victimization and school connectedness) across four racial/ethnic groups––Asian, Black, Latinx, and White students. Results suggested that a balanced racial/ethnic representation reduced racial/ethnic disparities in race-based victimization. Study 2 investigated how the teacher and student racial/ethnic diversity were uniquely related to students’ perception of school climate by intersecting identities between socioeconomic and racial/ethnic identities among White and Latinx students. The results showed that among Latinx and White students with low socioeconomic levels, greater teacher racial/ethnic diversity was associated with more positive reported school attitudes in parental engagement and perceived school equality and safety. Study 3 discussed concerns about using the existing diversity measures and operationalizations of three distinct diversity concepts: normic, dual-concept, and representative. Additionally, it proposed alternative mathematical expressions to capture dual-concept diversity and underscored some of the conceptual ambiguities of current research in measuring and applying different diversity concepts. The collective findings imply that maintaining a power balance is vital to protecting students from social exclusion experiences; efforts in facilitating positive psychosocial experiences are particularly needed for schools with diverse student populations and students with historically marginalized identities. More research is also warranted in diversity measures and conceptualizations. Practical implications for fostering psychosocial experiences in a diverse school context are discussed.