Research has shown the importance of teacher leaders in improving professional learning and schools. This case study investigates the development of a new teacher leader position at a Norwegian upper-secondary school with the responsibility for leading peer teachers' collective professional development. Our aim is to understand the interplay between cultural/structural conditions and dynamic processes in the practice of leading one's peers. Our findings show that teacher leaders and their practices switch between leading and reflecting on equal terms with their peers and by showing loyalty to the teaching profession. We argue that establishing a teacher leader position has the potential to change and develop teachers' practices from individual to increased collective professionalism.