The divergent roles of social media in adolescents' academic performance have not been confirmed, as previous studies failed to address social media use in different contexts. This study thus aims to explore the relationship between outside and inside social media behavior and academic performance in Chinese adolescents. Altogether, 560 Hong Kong adolescents (47.0% girls) were recruited and surveyed with Outside School Social Media Behavior (OSSMB) and Inside School Social Media Behavior (ISSMB). Their impulsivity and academic performances were also evaluated. Linear regression analysis and structural equation modeling (SEM) results jointly indicated that: (1) OSSMB negatively predicted the adolescents' academic performance, whereas ISSMB positively predicted their performance; (2) the two subdimensions of ISSMB -- the consuming and sharing behaviors -- positively predicted academic performance; and (3) ISSMB and impulsivity played multiple mediation roles in the relationship between OSSMB and academic achievement. The results also suggested that the relationship between outside school social media behavior and academic performance may be undermined by the opposing mediation effects of inside school social media behavior and impulsivity.