We document a largely unrecognized pathway through which schools promote human capital development -- by fostering informal mentoring relationships between students and school personnel. Using longitudinal data from a large, nationally representative sample of adolescents, we explore the frequency, nature, and consequences of school-based natural mentorships. Estimates across a range of fixed effect (FE) specifications, including student FE and twins FE models, consistently show that students with school-based mentors achieve greater academic success and higher levels of post-secondary attainment. These apparent benefits are evident for students across a wide range of backgrounds but are largest for students of lower socioeconomic status.