In sufficiently large schools, lessons are given to classes in sections of various sizes, depending on the subject taught. Consequently, classes have to be split into various given numbers of sections. We focus on how to subdivide a class in subgroups, so as to be able to reproduce all required sections by merging subgroups together, while minimizing the number of edges in the resulting course conflict graph. As a main result, we show that subdividing the students set in a regular way is optimal. We then discuss our solution uniqueness and feasibility, as well as practical issues concerning teacher assignments to sections.