Student-led courses have been described as a promising approach to improve sustainability education. However, there is a lack of systematic studies about the benefits and challenges of such courses. This qualitative case study examines the challenges and tensions that arose in the planning of a student-led higher education course on sustainability education. The challenges were identified from the student course designers’ conversations and interviews using discourse analysis, focusing on their disagreements during planning. The identified challenges concerned sustainability and sustainability education, the course designers’ roles, and collaborative decision-making. To relate the challenges to wider discourses on these topics, five underlying tensions were recognized. These include the tension between participatory action and critical discussion approaches for sustainability education, and the tension between drive towards unanimity and agreeing to disagree in collaborative educational planning. Finally, it is discussed how, and to what degree, the challenges and tensions can and should be mitigated.