Curriculum design is a domain that infrequently forms a discrete element of initial teacher training, or continuing professional development for music teachers in English secondary schools. Classroom music teachers, teaching Key Stage 3 (KS3) learners (11–14-year olds), are, however, required to design their own curriculum. Teachers are accountable to school leaders for curricula they implement, and the outcomes from their selected approaches. This article discusses classroom music curriculum practices and their significance in music education. The research project was conducted with music teachers from schools in the East and West Midlands of England, utilising questionnaires and semi-structured interviews. Research findings are conceptualised in a model of music curriculum activity. The research thereby facilitates discussion of unacknowledged complexities and tacit teacher planning practices in music curriculum design.