Although most teachers realize the seriousness of bullying and try to intervene, some take a passive stance, while others take active action. The present study examined individual and environmental factors that make teachers either passive or active interveners. Self-reported questionnaires were collected from 200 middle school teachers in South Korea. Teachers were classified into either active (54.5%) or passive action clusters according to differences in individual (learned helplessness, attributional style, preparedness) and environmental (social support) factors. In each cluster, the moderation effects of social support between teacher preparedness and bullying intervention were analyzed. Results indicate that adequate social support from school administrators is critical for passive teachers. However, in the case of active teachers, social support from colleagues was the most important moderator. For both groups of teachers, preparedness in bullying is necessary to increase their bullying intervention. The results indicate that the implementation of differential strategies for each teacher cluster is key to promoting bullying intervention.