In a school choice context we show that considering only schools' priorities and the set of acceptable schools for each student - but not how these schools are ranked in their preferences - we can restrict the set of possible stable matchings that can arise for any preference profile of the students that leaves the set of acceptable schools unchanged. We characterize impossible matches, i.e., of pairs student school that cannot be matched at any stable matching, for any preference profile. Our approach consists of linking Hall's marriage condition to stable matchings. Our results offer a new methodology to assess to what extent the preferences on one side of a matching market can preset the stable matchings that can emerge. First, we use this technique to discuss the impact of priority zoning in school choice problems. Second, a new mechanism for school choice problems is proposed. It is shown that it (weakly) Pareto dominates the Student Optimal Stable Mechanism and retain some of its incentives.
