Whereas school-based prevention programs often target deficits in individual children's social skills in order to limit their aggression or exposure to peer victimization, there is increasing evidence that school-wide and classroom-level factors can also affect the success of these programs. This short-term longitudinal study involved 432 elementary school students from 44 classrooms in 17 urban schools. We investigated whether classroom characteristics (average levels of social competence, emotional problems, and behavioral problems) and school-wide characteristics (proportion of children on income assistance and implementation of a peer victimization prevention program—the Walk away, Ignore, Talk, and Seek help [W.I.T.S.] program) experienced in Grade 1 influences changes in children's reports of relational and physical victimization at the end of Grade 2. Findings showed that classroom levels of emotional problems predicted increases in relational victimization (beyond individual differences in emotional and behavioral problems). Classroom levels of behavioral problems predicted reports of increases in physical victimization (beyond individual differences). Classroom levels of social competence also interacted with individual levels of emotional problems such that children with higher levels of emotional problems in classes with more socially competent children reported more relational and physical victimization. Higher school levels of poverty and lack of program involvement also predicted higher levels of physical victimization, beyond individual and classroom effects. The capacity of the W.I.T.S. program to influence classroom level characteristics and the moderating effects of school poverty on victimization were also assessed. © 2003 Wiley Periodicals, Inc. J Comm Psychol 31: 397–418, 2003.