This article addresses the strongest variables related to bullying, taking into account personal background, personality characteristics and social context. A survey of self-reported juvenile delinquency among more than 4500 pupils in the 8th and 9th grades was conducted in a French-speaking part of Switzerland. Offenders are more often boys, and boys score higher than girls for all the considered behaviours. The multivariate analyses showed that youths with a low level of self-control are more likely to bully others. Moreover, adopting a positive attitude towards violence and spending time with peers also increase the risk of bullying. The importance of environmental variables was also shown; youths are less likely to bully with increased supervision and monitoring by adults, and when schools participate in social control by impartially intervening to increase respect for the rules. Furthermore, when the school climate is considered in a positive light by the pupils, then their risk of committing bullying decreases. Finally, it appears that school absenteeism is a good indicator for detecting at-risk youths. The identification of these risk factors can be used to define appropriate intervention strategies.