The middle school transition is a salient developmental experience impacting adolescents around the world. This study employed a randomized-controlled treatment design, with randomization at the school level, to investigate the impact of a school adjustment program for middle school transition and potential gender differences. Participants included 1147 students (M
age = 9.62; SD = 0.30, 45.7 % girls), who were assessed at four time points during the transition, regarding five dimensions of self-concept (academic, social, emotional, physical and family) and self-esteem. Parallel growth curves were employed to analyze the evolution of self-concept. Following the transition to middle school, students reported lower levels of self-concept (academic, emotional and physical) and self-esteem, while participation in the intervention led to increases in self-esteem and gains in social self-concept. No gender differences were found. These results provide preliminary evidence supporting such interventions in early middle school transitions.