The meaning of the term 'inclusion' is often taken for granted and seldom defined. Empirical research on inclusive education is often normative since it is based on terms such as 'justice' and 'democracy'. Such terms are challenging to translate into real practice because their meanings depend on a subjective evaluation related to the time and place where inclusion is supposed to happen. Inclusive education, therefore, is challenging to explore in research and to achieve in educational situations. This article explores the understanding of inclusive education through the lens of social system theory developed by Niklas Luhmann as well as theory of institutionalism. With the perspectives underlying mechanisms that create inclusion and exclusion in schools are identified at different institutional levels. Furthermore it is shown how subsystems include and exclude, i.e. what criteria apply to the access and rejection of a system. In this theoretical contribution to understanding inclusive education, we seek to intertwine Luhmann's theory of inclusion and exclusion with the institutional theory of the social construction of reality to discuss how policy, management, teaching, student relationships, and everything within the context of education that involves communication can create institutionalised systems with mechanisms that form persistent exclusion for some students.