Electronic social communication has provided a new context for children to bully and harass their peers and it is clear that cyberbullying is a growing public health concern in the US and abroad. The present study examined individual and contextual predictors of cyberbullying in a sample of 16, 634 students in grades 3-5 and 7-8. Data were obtained from a large cluster-randomized trial of the KiVa antibullying program that occurred in Finland between 2007 and 2009. Students completed measures at pre-intervention assessing provictim attitudes (defined as children's beliefs that bullying is unacceptable, victims are acceptable, and defending victims is valued), perceptions of teachers' ability to intervene in bullying, and cyberbullying behavior. Students with higher scores on provictim attitudes reported lower frequencies of cyberbullying. This relationship was true for individual provictim attitudes as well as the collective attitudes of students within classrooms. Teachers' ability to intervene assessed at the classroom level was a unique, positive predictor of cyberbullying. Classrooms in which students collectively considered their teacher as capable of intervening to stop bullying had higher mean levels of cyberbullying frequency. Our findings suggest that cyberbullying and other indirect or covert forms of bullying may be more prevalent in classrooms where students collectively perceive their teacher's ability to intervene in bullying as high. We found no evidence that individual or contextual effects were conditional on age or gender. Implications for research and practice are discussed.