We present a case study that examined the role of teacher belief change and agency development through enactment of an innovative student-centered STEM program in three public urban secondary schools. In the program, 9th grade STEM classrooms were restructured to incorporate daily small group instruction facilitated by 10th grade near-peers. We found that teachers who had successfully persisted in the program for several years attributed changes in their professional practices, principles, and norms to their experiences with students. Teacher agency developed as teachers reflected on reform-based experiences for planning, and as they further conceptualized their own authority to distribute it among their students. All teachers exhibited qualities of deep change, exerted agency in their own classrooms, and perceived they had school-level cultural resources to spread agentic actions beyond their classrooms, but not all teachers felt agentic in relationship to social structures in their schools. Through our findings, we identify perceptions about the student experience as a salient source of information that motivates teacher belief change. We further show how cultural and social-structural resources afford or hinder teachers from sustaining and scaling a relatively mature program.