The past few decades have witnessed a global enforcement of "governance by data" in education policy, including a significant increase of assessments and quantified evaluation. Within this context, this article focuses particularly on the intensifying evolvement of new (digital) information technologies and "mediated" infrastructures of data flows. The premise is that such technologies and actors of mediation reveal a crucial potential for implementing a new mode of digitalized governmentality in education, which, as "governance by big data", reaches far beyond policy, into educational administration, school practice and individual learning activities. The strategic mediation of (big) data in education (such as that generated through assessments) involves actors, structures and technologies that operate "between" policy, politics, administration, schools and individuals as well as "between" data production, consumption and the data itself, for example by applying practices of data visualization or technical data services around software and databases. However, as this article seeks to demonstrate, such mediators comprise various types of actors who operate very differently within the diverse sectors of education policy, indicating a highly ambiguous yet powerful composition of digitalized governmentality.