Niklas Luhmann's late concept of inclusion/exclusion in world society compensates for the lack of a systems theory of social differentiation, but ultimately remains incomplete. Moreover, it is remarkable that Luhmann apparently never linked the inclusion/exclusion concept to his late theory of the education system. The following considerations seek this missing link, assuming that both functional and social differentiation constitute a double-sided form of societal structuring generated by the polycontextural operation of social closure mechanisms. The term 'educational closure' refers to the autopoietic form of inclusion/exclusion processed by an education system internally coupling operational and social closure. The article argues that organised education worldwide operates as an observing system autopoietically generating categorical differences, as well as inequalities between persons. While grouping, sorting, and tracking pupils, the education system seeks to maintain the recursion of educational communication, but also generates unequal social differentiation as a far-reaching side effect. Finally, reconstructing Weber's theory of social closure as an earlier inclusion/exclusion concept may provide a theoretical basis for further empirical research into the social consequences of educational inclusion/exclusion in a world society.