In this contribution, the expansion of education is analyzed in terms of its institutional characteristics rather than its quantitative characteristics (more students, more schools, more teachers, longer educational careers. The differentiation of the middle secondary level is its primary interest. On the one hand, the background is illuminated; on the other hand, the process of differentiation of the system for Gymnasiums as well as trade schools is reconstructed. In both sections of the educational system, the expansion is interconnected with various differentiations of the system. To conclude, it becomes apparent that we are dealing with the phenomenon of expansion through the differentiation of the system that is a general sign of growth in the educational system, which does not merely concern the 60's and 70's or just the middle secondary school level.