The period of early adolescence is characterized by dramatic changes, simultaneously affecting physiological, psychological, social, and cognitive development. The physical transition from elementary to middle school can exacerbate the stress and adversity experienced during this critical life stage. Middle school students often struggle to find social and emotional support, and many students experience a decreased sense of belonging in school, diverting students from promising academic and career trajectories. Drawing on psychological insights for promoting belonging, we fielded a brief intervention designed to help students reappraise concerns about fitting in at the start of middle school as both temporary and normal. We conducted the first district-wide double-blind experimental study of this approach with middle-school students (N=1,304). Compared to the control condition activities, the intervention reduced sixth-grade disciplinary incidents across the district by 34%, increased attendance by 12%, and reduced the number of failing grades by 18%. Differences in benefits across demographic groups were not statistically significant but some impacts were descriptively larger for historically underserved minority students and boys. A mediational analysis suggested 80% of long-term intervention effects on students' GPA were accounted for by changes in students' attitudes and behaviors. These results demonstrate the long-term benefits of psychologically reappraising stressful experiences during critical transitions and the psychological and behavioral mechanisms that support them. Furthermore, this brief intervention is a highly cost-effective and scalable approach that schools may use to help address the troubling decline in positive attitudes and academic outcomes typically accompanying adolescence and the middle school transition. [This paper was published in "Proceeedings of the National Academy of Sciences" v116 p16286-16291 2019.]