Students' sense of social relatedness at school predicts health and well-being throughout life. However, little is known about whether observed associations reflect unobserved family background factors and whether these associations differ between student-student and student-teacher relationships. Using data from the National Longitudinal Study of Adolescent to Adult Health, this study examined whether student-student and student-teacher relationships are differentially associated with adult health outcomes, measured by self-reported overall health, physical health, psychological health, and substance use. This study employed sibling fixed-effect models to take into account unobserved family background factors such as genetic endowments, family environment, as well as childhood social contexts (school and neighborhood effects). Naive ordinary least squares (OLS) models showed significant associations between relationships with other students and health outcomes in adulthood. However, the preferred sibling fixed-effect estimates revealed that family background characteristics confound these observed associations, with the exception of the depression outcome. Conversely, observed associations between adolescents' relationships with teachers and adult health were robust to controlling for unobserved family background characteristics shared between siblings. Taken together, improving the quality of social relationships in schools, especially student-teacher relationships, may improve adult health in the long run.