Using data from the Early Childhood Longitudinal Study--Kindergarten cohort (ECLS-K), this study aimed to identify body mass index (BMI) trajectories from kindergarten to eighth grade using group-based trajectory modeling (GBTM), examine the relationships between BMI trajectories and adolescents' school achievement (reading and math), determine whether parental warmth had a protective effect on adolescents in the higher BMI groups, and assess whether similar patterns were observed for boys and girls. Three distinct BMI trajectories were identified. Adolescents in the gradually increasing BMI group and consistently higher BMI group were shown to have worse academic achievement compared with adolescents in the consistently low-to-normal BMI group. Parental warmth in eighth grade played a protective role for girls but not for boys, suggesting important gender differences in the role of parental warmth in overweight and obese adolescents' school achievement. Our results highlighted the importance of intervention programs tailored for specific subgroups of overweight and obese adolescents.