School systems may be usefully characterized according to Turner's proposed ideal types of sponsored and contest mobility. Germany is a critical case with respect to this typology because its secondary school system is stratified and selective, and yet it offers the opportunity for upward and downward mobility. Drawing on an analysis of a German longitudinal dataset, this paper addresses the question of flexibility or rigidity of the school system, exploring the ways in which factors other than pupils' ability influence selection processes within that system. Both academic ability and ascriptive factors act together to facilitate or hinder changes of academic routes within the school system. The methodological focus of the paper is on the introduction to an innovative method, Charles Ragin's Qualitative Comparative Analysis, a method based on set theory. It involves the identification of necessary and sufficient conditions for a given outcome, taking conjunctions of causal conditions into account. (Contains 2 figures, 12 tables, and 11 notes.)