The structure of priority in the school choice problem

In a school choice problem, each school has a priority ordering over the set of students. These orderings depend on criteria such as whether a student lives within walking distance or has a sibling at the school. A priority ordering provides a ranking of students but nothing more. I argue that this information is sufficient when priority is based on merit but not when priority is based on criteria such as walking distance. I propose an extended formulation of the problem wherein a 'priority matrix', indicating which criteria are satisfied by each student-school pair, replaces the usual priority orderings.