Instruction and training in social and emotional learning (SEL) is an important component in addressing the emotional and behavioral needs of students. This study is the first to examine whether Strong Kids, an SEL program, delivered school-wide in all classrooms, could result in decreased internalizing behaviors and increased prosocial behaviors for both at-risk and general education students. This study also evaluated whether teachers could implement Strong Kids as designed and whether they viewed it as socially valid. A non-equivalent control group design was used. The treatment school consisted of 348 student participants and 17 teacher participants in grades K–6. The control school consisted of 266 student participants and 11 teacher participants in grades K–6. Pretest and posttest teacher ratings revealed significant decreases in students’ internalizing behaviors at the treatment school, while these behaviors increased at the control school. Students at risk for emotional and behavioral disorders improved significantly more than those not at risk on ratings of internalizing symptoms and prosocial behaviors, even when the intervention was delivered school-wide in all classrooms. Teachers were able to implement Strong Kids with a high level of fidelity and they held moderately favorable views of the program.