This study investigates three important aspects of the classroom context in shaping adolescents' classroom engagement trajectories: (a) teacher support, (b) peer norms (i.e., descriptive and popularity norms), and (c) ethnic classroom composition (i.e., ethnic heterogeneity and proportion of majorities). An ethnically diverse sample of 730 adolescents from Grades 9 to 11 was followed annually. Longitudinal multilevel models revealed that more teacher support and higher classroom-levels of engagement (i.e., descriptive norms) promote adolescents' behavioral and emotional engagement. Moreover, more ethnic heterogeneity in the classroom related to less steep decreases in behavioral engagement over time, whereas higher proportions of majorities in the classroom were associated with steeper decreases in emotional engagement over time. Associations were the same for ethnic minorities and majorities. Furthermore, teacher support and descriptive norms jointly buffered against declining behavioral engagement trajectories. In general, this study underscored the importance of the classroom context in adolescents' behavioral and emotional engagement.