In his posthumously published work on education, Niklas Luhmann (1927-1998) described his approach to the sociological observation of the educational system as a formal one. Only by refraining from more substantial definitions, based on what teachers hope to convey, he claimed, can education be understood in all its historical and geographical varieties. By rereading his oeuvre as a socio-historical account of modern school education's morphogenesis, I inspect this so-called formal approach more closely and show how it brings together different understandings of its central notion, form. The underlying theoretical movements of Luhmann's formal approach, composed of indication and delimitation, of generalisation and re-specification, provide useful hints for the study of those forms of education which today increasingly emerge outside of its formal institutions.