External and standardized assessments based on student results are a contested education policy among school actors. Movements of opposition have emerged in different countries, especially in those contexts with high-stakes accountability systems. However, this phenomenon has not been analyzed in soft accountability systems. The objective of this article is to study the opt-out movement in Catalonia, understood as an anti-standardization movement in a system of soft accountability. In order to do so, we adopt the case study approach as a methodological strategy, based on the triangulation of semi-structured interviews with activists (n = 14), key stakeholders (n = 3), and document and press analysis (n = 25). The results shed light on the emergence and nature of the movement, its opportunity structures, the discursive frames and the repertoires of collective action. Our results show how accountability instruments have a 'life of their own' beyond their policy design. In this sense, the opt-out movement in Catalonia identifies potential risks and adverse effects similar to those reported in high-stakes systems, developing a repertoire of collective action and discursive frames similar to other emerging anti-standardization movements in high-stakes contexts.