Prior research highlights the mismatch between adolescents' growing capacities for autonomy and the limited opportunities for influence in U.S. secondary schools. Youth-led participatory research (YPAR), an approach in which young people research and advocate for change on problems of concern to them, could increase students' autonomy in secondary schools. This qualitative study of YPAR examined whether and how the intervention meaningfully affected the interactions and roles of students and adults in two distinctive urban high school settings, identifying concepts for further empirical investigation. Results suggested that YPAR enabled processes of student professionalization that led to novel student-adult "collegial" interactions, expansion of domains of student influence, and diversification of students with opportunities to influence policies and practices across these two schools. (Contains 2 tables and 2 figures.)