This longitudinal study examines how the time that youth spend in activities during high school may contribute to positive or negative development in adolescence and in early adulthood. We draw on data from 1,103 participants in the longitudinal Youth Development Study (Minnesota), followed from entry to high school to their mid-20s. Controlling demographic, socio-economic and psychological influences, we estimate the effects of average time spent on homework, in extracurricular activities and with friends during the four years of high school on outcomes measured in the final year of high school and 12 years after the start. Our results suggest that policies surrounding the implementation and practice of homework may have long-term benefits for struggling students. In contrast, time spent with peers on weeknights was associated with both short- and long-term maladjustment.