The purpose of this article is to contribute to existing literature about how activity theory might be used in music education research. It draws from the author's doctoral action research into the assessment of group composing for New Zealand's secondary school qualification, the National Certificate of Educational Achievement (NCEA). It outlines and explains how activity theory was used to examine three interacting activities: social jamming, group composing and achievement in the NCEA. Analysis revealed a relationship between students' identities, their achievement in NCEA group composing, and socioeconomic disparity.