Social-emotional learning programs are increasingly being implemented in U.S. schools to address a wide range of problematic behaviors (e.g., bullying, delinquency) and to promote academic success. The current study examined the direct and indirect impact of the Second Step Middle School Program (Committee for Children, 2008) on bullying, cyberbullying, homophobic name-calling, and sexual harassment perpetration over the course of a 3-year randomized clinical trial. Delinquency was examined as an intervening variable between treatment condition and aggression outcomes. Thirty-six schools in Kansas and Illinois were assigned to either a Second Step condition or a control condition, and 3,651 sixth-grade students completed self-reported surveys at four time points across 3 years. Students in the Second Step condition received a total of 41 lessons across the 3-year study. No direct intervention effects were found for multiple forms of aggression perpetration at the end of 3 years. However, as hypothesized, decreases in self-reported delinquency (intervening variable) over the first 2 years were significantly related to decreases in bullying, cyberbullying, and homophobic name-calling perpetration for Second Step schools across the 3-year study. Indirect effects of the Second Step program on bullying and aggressive behavior were statistically significant through reductions of delinquency.