Meyer and Land's work (among subsequent others) on threshold concepts (TCs) has been influential in numerous subjects, particularly in higher education. However, despite its growing international interest, its application into the domain of music in schools is a highly under-researched area. This article draws on the notion of TCs focusing on the context of lower-secondary school (Key Stage 3: ages 11-14) group composing. Using video-recorded and interview data from three case-study schools in the English Midlands, examples of TCs are presented and how formative assessment was, or could have been, a key process in them being crossed is discussed.