The concept of 'knowledge' is central to education, particularly when it comes to the school curriculum. This paper makes the case for engaging in conceptual analysis, and in particularly conceptual engineering, in educational studies. Conceptual engineering emphasises analysing concepts with their purposes in mind. To illustrate the importance of this sort of conceptual analysis in education, I track the educational impact of three concepts of knowledge: (1) the traditional philosophical concept of knowledge as justified, true belief; (2) Michael Young's concept of 'knowledge of the powerful'; and (3) Young's concept of 'powerful knowledge'. I argue that knowledge as justified, true belief is too abstracted from educational purposes to provide much guidance for curriculum planning. Young's 'knowledge of the powerful' prioritises the social role that knowledge can play over its value to individual students. Finally, 'powerful knowledge' is too unclear to provide appropriate guidance to policy makers and practitioners. By examining the interactions between these concepts of knowledge and historical policy and practice in education, I conclude that the focus of conceptual engineering on clarity about the purposes of education is central to good conceptual analysis.