Objective: To evaluate the benefits of the Fast Track Friendship Group program implemented as a stand-alone school-based intervention on the social cognitions, social behavior, peer and teacher relationships of peer-rejected students. Method: Over four successive years, 224 peer-rejected elementary students (57% White, 17% Black, 20% Latinx, 5% multiracial; 68% male; grades 1- 4; M[subscript age] = 8.1 years old) were identified using peer sociometric nominations and randomly assigned to the intervention (n = 110) or a treatment-as-usual control group (n = 114). Four school districts serving economically-disadvantaged urban and rural communities participated. Intervention involved weekly small group social skills training with classmate partners, with sessions tailored to address individual child needs. Consultation meetings held at the start and mid-point of intervention were designed to help teachers and parents support the generalization of targeted skills. Results: Multi-level linear models, with children nested within schools (controlling for demographics and baseline scores) documented improvements in social-cognitive skills (direct assessments of emotion recognition and competent social problem-solving), social behavior (teacher ratings of social skills and externalizing behavior), and interpersonal relationships (peer sociometric nominations of peer acceptance and friendships, teacher-rated student-teacher closeness). Significant effects were generally small (ds = 0.19 to 0.36) but consistent across child sex, grade level, and behavioral characteristics. Conclusions: The intervention proved feasible for high-fidelity implementation in school settings and produced significant improvements in the social adjustment of peer-rejected children, validating the approach as a school-based Tier 2 intervention. [This paper will be published in "Journal of Clinical Child and Adolescent Psychology."]