The promotion of social and emotional learning (SEL) in schools may help prevent emotional and behavioral problems of students. This study evaluated the effects of a SEL curriculum, Strong Start, on the social-emotional competence of 26 second grade students, using a quasi-experimental, non-equivalent control group design. Results revealed statistically significant and meaningful improvements in teacher ratings of students’ internalizing and peer-related prosocial behaviors, particularly for students at greater risk. Conversely, control group students experienced significant worsening of internalizing behaviors and decreased levels of peer-related prosocial behaviors. No changes were reported in externalizing behaviors for either group. Treatment integrity and social validity ratings of Strong Start were high. Limitations and implications of this study are addressed.