Research on students' learning in computing typically investigates how to enable individuals to develop concepts and skills, yet many forms of computing education, from peer instruction to robotics competitions, involve group work in which understanding may not be entirely locatable within individuals' minds. We need theories and methods that allow us to understand learning in cognitive systems: culturally and historically situated groups of students, teachers, and tools. Accordingly, we draw on Hutchins' Distributed Cognition [16] theory to present a qualitative case study analysis of interaction and learning within a small group of middle school students programming computer music. Our analysis shows how a system of students, teachers, and tools, working in a music classroom, is able to accomplish conceptually demanding computer music programming. We show how the system does this by 1) collectively drawing on individuals' knowledge, 2) using the physical and virtual affordances of different tools to organize work, externalize knowledge, and create new demands for problem solving, and 3) reconfiguring relationships between individuals and tools over time as the focus of problem solving changes. We discuss the implications of this perspective for research on teaching, learning and assessment in computing.