Cultural-Historical Activity Theory has contributed significantly to studying and promoting educational change. Its distinctive concepts inform an approach to interventionist research called the Change Laboratory This paper reports on a Change Lab in an Australian secondary school resulting in major changes for students studying for the Higher School Certificate (HSC) in the final two years of high school. It reports on the key conflict between the HSC as a ticket to university admission (exchange value) and its importance for students' broader development and future (use value). Expansive learning and systemic contradictions have been frequent reference points in reports of school-based Change Laboratories but more recent developments in theorising transformative agency by double stimulation have received less attention. Addressing this gap, this paper considers the construction and use of artefacts that helped participants understand the problem, and develop previously unknown solutions that resulted in a qualitative transformation of activity. Mirror materials comprising aggregate and individualised student data served as a first stimulus; four-field representations of collective zone of proximal development and germ models functioned as higher abstraction second stimuli, complemented by teacher-generated ideas and models. Changes made by the school transcended the central contradiction in the local activity system, providing valuable insights into the process of change amid widespread critiques of reductive measures of student outcomes at the end of secondary school.