As students transition to post-secondary education, they experience considerable stress and declines in academic performance. Perceived social support is thought to improve academic achievement by reducing stress. Longitudinal designs with three or more waves are needed in this area because they permit stronger causal inferences and help disentangle the direction of relationships. This study uses a cross-lagged panel and a bivariate growth curve analysis with a three-wave longitudinal design. Participants include 10,445 students (56% female; 12.6% born outside of Canada) transitioning to post-secondary education from ages 15-19. Self-report measures of academic achievement and a generalized measure of perceived social support were used. An increase in average relative standing in academic achievement predicted an increase in average relative standing on perceived social support 2 years later, but the reverse was not true. High levels of perceived social support at age 15 did not protect against declines in academic achievement over time. In sum, perceived social support appears to have no bearing on adolescents' future academic performance, despite commonly held assumptions of its importance.