Despite promising evidence that school-based health centers (SBHCs) and school mental health (SMH) services can reduce disparities in access to mental health care for underserved populations, little is known about what predicts retention of youth in SMH services. This study aimed to identify individual- and school-level factors associated with access and retention in mental health services provided in SBHCs. Existing data were analyzed for 17,348 students (2205 who received SMH services) who attended 17 middle and high schools in an urban school district. Student-level predictors included sex, race/ethnicity, and mental health need at intake. School-level predictors included grade level (middle vs. high school), student/teacher ratio, percent of white students, and percent of students receiving free or reduced lunch. Hierarchical generalized linear modeling was used to examine the association between student- and school-level variables and retention in care. White, Latino, and Asian students were less likely to access SMH services; multiracial and Native American students were more likely to access services, and African-American students were no more or less likely. Hierarchical models found that retention in services was more likely for female students, those who presented with social/educational problems, internalizing problems, and trauma/abuse/neglect, and less likely for Asian students. Retention was also more likely for students in middle schools, schools with a lower student/teacher ratio, and schools with a smaller percentage of students eligible for free or reduced lunch. While SMH services show promise in increasing access for underserved groups, ensuring equitable retention of students in care will likely require concerted effort focused on consistently engaging students and ensuring quality of care.