The current studies explored (a) the extended external validity of social-goal-orientation framework; (b) the mediating role of social goals between classroom goal structures and students' engagement; and (c) whether changes in social goals can be explained by classroom goal structures and engagement. Study 1 was cross-sectional (N = 317), and study 2 included two time points, with a 6-month gap (N = 223), among sixth-grade students. The findings indicated that mastery goal structure was associated with social-development goals and engagement, whereas performance goal structure was associated with demonstration. Cross-lagged analysis revealed that (a) social goals are relatively stable; (b) development goals positively predicted change in emotional engagement, and (3) behavioral engagement positively predicted changes in development goals and negatively predicted changes in demonstration-avoidance goals.