There is now an extensive body of research concerning the OECD's Programme for International Student Assessment (PISA), and its recent additions and variations. Despite this, we argue that there is a paucity of theory-informed empirical research on the ways in which the PISA results about a nation's education system both circulate within, and are engaged by, educators and policymakers at different levels of the education system. This paper reports on a large multi-scalar, multi-method and multi-actor dataset on the Mexican education system, where we explore what actors located at different scales know about PISA, and whether and how they use it in education policy and practice decisions. From a set of different vantage points we show the highly contingent and unpredictable 'life of numbers' -- from teachers in schools who barely know about and engage with PISA data, to politicians who use PISA to legitimate their own political purposes, or policymakers who draw upon much earlier renditions of PISA so that it now enters into Mexican education policymaking and shapes practices through a metaphoric back door. We reflect theoretically on these processes and practices, arguing for a more complex and nuanced reading of large-scale assessments like PISA.