The current study examined the effects of a community-based, whole-school peer victimization prevention program (WITS Primary Program), implemented from Grades 1 to 3, on trajectories of child-reported peer victimization and help-seeking and teacher-reported social-emotional adjustment from Grades 1 to 6. This quasi-experimental design followed a cohort of 432 children in 11 program and 6 comparison public elementary schools over 6 years. There were significant and meaningful effects of the WITS Primary Program on linear changes in physical victimization (Cohen's d = 0.17), relational victimization (Cohen's d = 0.20), and social competence (Cohen's d = 0.20); significant and small effects on physical aggression (Cohen's d = 0.09); and nonsignificant effects for help seeking and internalizing (Cohen's d values = 0.04 and 0.10, respectively) during elementary school. Following the transition into middle school, the program effects faded, with the exception of some subgroups in high-risk contexts. Our findings suggest that peer victimization prevention programming implemented in early elementary school may need to be sustained to maintain the promising early intervention gains through the transition into middle school. (Contains 7 figures and 3 tables.)