Involvement in out-of-school activities, such as sports and clubs, is linked to greater intrinsic motivation and positive mood in children and adolescents, as well as opportunities for social skill development. However, the impact of out-of-school time (OST) activities varies with youths' perceptions of their quality, and few studies have examined these effects longitudinally. While higher quality OST relates to improved social skills, children with stronger social skills tend to receive more positive responses from their environments. In this brief report, we explored whether quality OST was linked to increases in social skills from early to middle adolescence and/or whether having social skills leads to higher quality OST experiences. In a sample of 996 mother--child dyads (45% female, 78% white, 24% low-income), we used cross-lagged path modeling to examine bidirectional relations between sixth-grade OST quality and ninth-grade social skills, as well as sixth-grade social skills and ninth-grade OST quality. The cross-lagged path model accounted for child sex, family income, father in the home, closeness with mothers, teachers, and peers, and number of OST activities. The cross-lagged effects were bidirectional: sixth-grade social skills were associated with increases in OST quality ([beta] = 0.08, p = 0.014), and sixth-grade OST quality was related to increases in social skills ([beta] = 0.10, p < 0.001). These findings have important implications for parents, teachers, and coaches who manage adolescents' extracurricular activities.