The present study investigates how Flemish middle school mathematics teachers make sense of schoolperformanfeedback data from low-stakes, external standardised tests. We take an in-depth look into the interpretive steps they take, based on a conceptual model that integrates intuitive and rational aspects of individual and collective sensemaking and empirical data collected in semi-structured interviews. We describe the nature of these sensemaking processes and consider the impact of influencing factors. Our findings demonstrate that the mere availability of school performance feedback data does not spontaneously spark sensemaking, nor does it necessarily lead to improvements in instructionalpractice. Teachers' sensemaking of schoolperformancefeedback data appears to be a largely intuitive process, grounded in external attributions and absent of triangulation. Challenges regarding expertise and lack of inquiry-based attitude and commitment result in superficial and often incorrect interpretations of the data that tend to remain uncorrected as teachers barely engage in collaborative professional dialogue about the data.