With the rise of accountability policies since the early nineties, the daily operation of English schools has profoundly changed. Through the in-depth analysis of ability grouping practices in one English secondary school, this paper aims to explore how the accountability shift and datafication impacted the practice of student grouping and students' experience of education. The paper documents how an English secondary school which had fully endorsed the comprehensive ideal gradually shifted from mixed ability teaching to a rigid system of hierarchically arranged attainment-based grouping structure over a period of decades, and explores the pedagogical dilemmas that data-driven governance generates. The paper concludes that the school's data-driven practice creates an environment of competition and an experience of incessant insecurities for the students which unsettlingly echo the culture of the late neoliberal labour market.