As of 2014, the State of Vermont adopted a policy package directing schools to implement a new way of educating and graduating students. Vermont's Act 77 and Education Quality Standards require schools to identify specific skills and knowledge students must be proficient in to graduate and ensure each student has a personalized experience. Vermont's tradition of local control means high school principals are leading change in their schools. How high school principals are understanding, valuing, and acting to lead the implementation of Vermont's policies for personalized, proficiency-based education therefore varies by location. The purpose of this multiple case study was to explore high school principal leadership at a critical juncture of implementing Vermont's personalized proficiency education paradigm. Principals' understanding was analyzed using an accountability vs. flexibility framework influenced by Sturgis (2016) and Labaree (1997). Principals' action steps were considered through the lens of Frontier and Rickabaugh's (2014) five levers for school change. Principals' leadership style was analyzed using transformational leadership practices (Leithwood & Sun, 2012). The findings from five high school principals suggest that Vermont's policies were valued for their potential to improve student outcomes, student engagement, and future career opportunities. Principals' understanding of the policies varied, suggesting a lack of cohesive understanding of policy goals across the state. They leveraged school structures and education standards to enact policy implementation, suggesting a focus on proficiency over personalization at the time of this study. Principals in this study were leading school change by building teacher capacity and enabling teacher collaboration, suggesting they were accessing key transformational leadership practices to enable school change. The principals also identified sticking points to implementation, including: a lack of appropriate computer software for proficiency-based grading, a limited timeframe for implementation, limited leadership resources and supports from the state, and worries about a lack of equity of opportunity for all students. This study has implications for education policy developers, state-level education agencies, and other high school principals. The findings suggest school-based implementation of state-level education policy, even in local control states, can be facilitated by supports for growing leader understanding of policy intentions and facilitating policy implementation with suggested timelines and key steps. The findings also suggest high school principals access transformational leadership practices when engaging their schools in a change process. By increasing the breadth of these practices, they can facilitate the whole-school change in a personalized proficiency paradigm.