The challenge for high schools to adopt effective measures to reduce bullying has been underscored by international media coverage highlighting the consequences of school bullying. Despite whole-school anti-bullying programs being accepted as the best evidence-based approaches to intervention, research continues to yield ambiguous findings, and only a limited number of studies have been conducted in secondary schools to systematically evaluate the components of this approach. The aim of this study was to investigate the effectiveness of anti-bullying interventions in four international secondary schools in Hong Kong. Schools were randomly assigned to: (a) a whole-school intervention, (b) a curriculum and Shared Concern intervention at Year 7, (c) a Shared Concern intervention at Year 7 and (d) a control school. Year 7 students in the four schools anonymously completed a bullying questionnaire at the beginning (N = 545) and end of the school year (N = 549). A highly significant main effect for schools was found between pre-test and post-test composite bullying scores (F = 7.70, p less than 0.001). Results showed the most significant reductions occurred when a whole-school intervention was used (F = 10.73, p less than 0.001). The research provides strong support for use of whole-school preventative/management interventions and the effective components of this approach are discussed.