Starting Strong in Kindergarten (Starting Strong) is a school-based indicated prevention targeting behavior problems, student-teacher relationships, and parent-school connectedness for children with or at risk for disruptive behavior problems during the transition to kindergarten. By use of a block-randomized, controlled trial to test program feasibility and efficacy, seven schools (representing 33 kindergarten classrooms and 97 children, 74% of whom were boys, 57% of whom were White, and 38% of whom were English language learners) were assigned to the immediate-intervention versus delayed control condition. A 10-week parent group (offered in both English and Spanish) and four-session teacher group consultation program addressed strategies for promoting positive behavior, student-teacher relationships, and parent-teacher collaboration. Feasibility was supported by high parent and teacher attendance, engagement, and satisfaction. The intervention had effects on overall parent- and teacher-reported behavior problems, as well as on parent-reported externalizing problems and parent- and teacher-reported internalizing problems, although not on a teacher-reported externalizing problems scale. Implications for sustainable, school-based mental health interventions are discussed. The transition to kindergarten may be a particularly apt time to address behavioral and relational adjustment in school.