Schools now face considerable pressure to be using educational data to inform decision-making and become more efficient. Key to this rise of the 'data-driven' school is the increased use of digital technologies -- with computer-based data processing fuelling hopes for the technology-driven 'smart schooling' and a general 'datafication' of education. Against this background, the present paper explores the realities how data-work is taking place within individual schools with the aid of digital technologies. In particular, it draws on empirical data generated from in-depth ethnographic studies of digital data practices within three secondary schools in the Australian state of Victoria. Using concepts from the sociology of numbers, the paper takers these empirical findings to develop productive and relational accounts of the predominant ways that data work was being carried out within these schools. In contrast to the idea of 'Big Data', schools were most often found to be engaging in small-scale, improvised forms of spreadsheet work. The paper considers the social, cultural and political reasons underpinning schools' reliance on such forms of 'small data' -- particularly in contrast to the more sophisticated, automated and continuous forms of data analysis that are now available.