Bullying, which includes cyberbullying and face-to-face bullying, is amongst the most pervasive threats to the wellbeing of children and young people. As social environments, schools are at the forefront of managing bullying behaviours. The rapidly changing and complex nature of bullying requires schools to put in place and maintain systems to prepare for and respond to such activities, and to continually test and refine these systems to ensure optimal performance. Despite this clear need, there is a lack of school level, self-assessment tools that enable schools to assess and measure their preparedness to deal with bullying and related disruptive activities. The aim of this paper was to describe the development, and reliability and validity testing, of such a tool--the School Self-Assessment Tool. The result is a 22-item, evidence-based, reliable, and validated instrument, situated within socio-ecological theory, and drawing on theories of behaviourism, social learning theory, prevention science, and systems change in the school climate literature. Schools may use the SSAT-22 in a number of ways depending on their needs, including to monitor progress, examine areas of strengths or challenges, and/or assist in collaborative efforts with other schools.