Education systems across the globe have enacted national testing regimes to monitor and report student achievement progress as an outcome of teaching performance. This paper reports on an investigation of strategies that Principals of high achieving schools use to achieve school results, based on NAPLAN reports (the National Assessment Program in Australia) and interpreted via the Alignment, Capability and Engagement (ACE) model of organisational readiness. Our findings identified specific Principal behaviours, actions and attitudes as necessary for effective school-wide improvement programs, as well as the existence of commonly shared strategies and approaches that help to explain why these particular Principals have been successful in their pursuit of school improvement. These include a shared vision for improvement, use of data-driven decision making, and building positive, "transparent" relationships to encourage teacher buy-in. Importantly, these findings identified "organisational readiness", a foundational principle of the ACE model, as a fundamental requisite to effective school improvement.