Friendships affect children's development and adjustment. This longitudinal study examined the possible heterogeneity and gender differences in the trajectories of children's friendship quality, as well as the relations between distinct friendship quality trajectories and school adjustment. A total of 3,779 Chinese elementary school students (45.5% girls, M[subscript age] = 9.94 years, SD = 0.72 at Time 1) completed measures of relevant constructs on five occasions at 6-month intervals. Latent class growth modeling revealed four heterogeneous developmental trajectory classes of friendship quality: high-increasing (49.6%), moderate-decreasing (5.1%), moderate-increasing (31.9%), and low-stable (13.4%). Girls were more likely than boys to fall into the higher friendship quality trajectory classes in reference to the low-stable class. Children in the high-increasing class and low-stable class showed the best and the worst school adjustment, respectively. The identification of four heterogeneous developmental trajectories of friendship quality with differential school adjustment outcomes highlights the importance of intervention programs tailored to specific groups for promoting children's friendship quality and school adjustment.