In an era of high accountability, teacher leadership is acknowledged as a key strategy for achieving school improvement. The current literature embraces shared leadership and the building of leadership capacity in schools, but does not sufficiently investigate learning cultures within program improvement schools as supportive of teacher leadership. This study considers teacher leadership as a possible means of building a shared culture of continuous improvement within program improvement secondary schools. The researcher investigated the culture of a program improvement year 5 school to understand the roles and functions of teacher leaders and to identify specific norms, habits, and structures that supported or inhibited the development of teacher leadership within this highly challenged environment. Using an explanatory mixed methods design, the researcher administered the Teacher Leadership School Survey (Katzenmeyer & Moller, 2001) within four program improvement secondary schools. Survey results, in combination with student achievement trends over three years, were utilized to identify the school that manifested the strongest presence of teacher leadership and the most significant progress towards exiting program improvement status. The researcher then utilized qualitative case study methodology to investigate this school in greater depth. Study findings support previous research that underscores the primary role principals play in creating conditions for active, ongoing teacher leadership. The study provided insights into the roles that teacher leaders assumed as they collaborated with the principal and their peers to address student performance-related challenges. Findings revealed a supportive school culture, largely situated within the context of professional learning communities. This culture engendered specific norms of practice that allowed teachers to lead in ways that positively influenced student learning and teacher practice. The researcher advances a Teacher Leadership Theoretical Framework to describe the ongoing dynamic of teacher leadership development and influence within program improvement secondary schools.