ABSTRACT Purpose Most research on tandem language learning (TLL) has involved university contexts . To contribute to the paucity of research in school settings, this study focuses on a high school teacher’s attempt at setting up an ESL-FSL exchange in a Canadian intranational context. To better understand the complexity of innovation, the process is analyzed through the lens of Activity Theory (AT). Methodology The study was designed as an action research project. To facilitate triangulation, three types of data were collected: 1. student questionnaires, 2. interviews with the teacher and six case study students, and 3. texts written by the case study students. The teacher reviewed the manuscript. Findings In line with AT, findings focused on how the teacher oriented to the TLL project and the tensions which arose during its implementation. Tensions were located both within the ESL activity system and between the ESL-FSL activity systems. Originality/Value To our knowledge, this study is the first to analyze the complexity of implementing TLL exchanges from an AT perspective. The study highlights how competing discourses within language teaching (multivocality) inhibited the implementation of the TLL project and foregrounds the role of transformative agency in regard to the teacher.