Student bullying behaviours are a significant social issue in schools worldwide. Whilst school staff have access to quality bullying prevention interventions, schools can face significant challenges implementing the whole-school approach required to address the complexity of these behaviours. This study aimed to understand how schools' capacity to implement whole-school bullying prevention interventions could be strengthened to promote sustainability and improve student outcomes. Qualitative methods were used to observe schools over time to gain insight into their implementation capacity to improve student social and emotional wellbeing and prevent and ameliorate harm from bullying. A four-year longitudinal, multi-site case study intensively followed eight schools' implementation of "Friendly Schools," an Australian evidenced-based whole-school bullying prevention intervention. Regular in-depth interviews with school leaders and implementation teams over four years led to the refinement of a staged-implementation process and capacity building tools and revealed four common drivers of implementation quality: (1) strong, committed leadership; (2) organisational structures, processes and resources; (3) staff competencies and commitment; and (4) translating evidence into local school policy and practice. This paper considers the strengths of qualitative data in understanding how and why bullying prevention interventions work as well as actions schools can take to enhance their implementation and sustainability of complex social interventions.