Over recent decades music composition has occupied an increasingly prominent place in international school music curricula with a consequent expansion of opportunities for students and of research into classroom-based composition learning and teaching. Concurrently, creativity researchers have investigated the lives and work of past and living eminent composers. This study of an orchestral composers’ school sits in the gap between research into school-age students’ composition and studies of eminent composers. For non-neophyte, but not yet expert, students, the professional workshop offers a prime space for learning. Drawing on expertise theory, this case study examines practices of expert composers working with advanced composer-students in an orchestral workshop environment. Observation and interview methods were used during and after the workshop to gain insight into outward manifestations of teaching and learning and into longer-term perceptions and reflections. This article focuses on one setting: a masterclass in which expert composers and students discussed issues emerging from the first rehearsal of students’ works. Findings suggest that, in this setting, teaching and learning strategies are characterized by modeling expert thinking, problem-finding, and sharing possibilities deemed promising based on experts’ accumulated experience. By sharing from their more extensive experience, expert composers led students to consider additional dimensions in compositional decision-making, extending students’ understandings of composing for orchestra to include not only nuanced technique, but also more insightful perceptions of professional orchestral culture.