Education is increasingly infiltrated by technology and datafication. This techno-data amplification is entangled with neoliberalism and the emphasis on calculation and measurement it brings, often through metrics. This article critically examines how metrics are shaping discipline practices in schools through ClassDojo, a popular platform for managing student behaviour. Little is known about how ClassDojo is implemented in schools, and how its dependence on metrics is impacting school discipline practices. Through a critical qualitative inquiry, we examined teachers' practices with ClassDojo, and found they operate via techniques of control, and that metrics are central to these techniques. We draw on the concept of 'metric power' to understand how these school discipline practices manifest as forms of power. We argue ClassDojo's metrics operate as powerful narrowing pedagogical devices that fixate on measurement and lead to practices which operate via neoliberal governing rationalities that reshape school discipline.