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This article analyzes co-assessment between teachers and students as a formative assessment approach that fosters learner engagement and reflexivity. Framed within the context of knowledge negotiation, co-assessment opens a dialogic space that transcends the limits of normative evaluation. An intervention-research project conducted in a French high school revealed a form of collective agency often invisible to both teachers and students. Its effects include a clearer understanding of learning issues for students and of assessment purposes for teachers. Co-assessment thus emerges as a lever for expansive and emancipatory learning, despite the persistent institutional constraints of traditional evaluation.