The goal of this study was to explore whether Strong Kids could result in improved social and emotional competence when implemented as a school-wide universal intervention. No prior studies have examined this question. This study also evaluated whether teachers could implement Strong Kids as it was designed and whether they viewed it as socially valid. It used a non-equivalent control group design. The treatment school in the study involved 348 students and 17 teachers from a Title I school. School demographics indicated that 61% of students were Hispanic, 37% White, and 2% of other ethnicities. Approximately 82% of the students qualified for free or reduced lunch. Teachers at the treatment school taught Strong Kids for 12 weeks, permitted treatment fidelity observations, and completed a social validity questionnaire (with a subgroup also participating in a social validity focus group). The control school participants consisted of 266 students and 11 teachers. The control school was selected because it was demographically similar to the treatment school. Teachers at both treatment and control schools completed pretest and posttest ratings of each of their students' internalizing behaviors and peer-related prosocial behaviors using nationally normed scales. Analyses comparing teacher ratings of the treatment school with ratings at the control school were performed using a split-plot ANOVA. Scores for students identified as at-risk through school-wide screening were compared to students not identified as at-risk. Average scores on the social validity questionnaire were calculated, and a qualitative analysis of the focus group was performed. Results revealed that 82% of lesson components were fully implemented. Teacher ratings at the treatment school reflected a significant decrease in students' internalizing behaviors, while ratings at the control school increased. At-risk students showed significantly greater improvements on both internalizing and peer-relations subscales compared to non-at-risk students. Social validity results revealed that Strong Kids provided a common language for teachers and students to talk about feelings and an avenue for students to seek help. It also helped teachers set school-wide expectations for handling social and emotional concerns.