Moderation Under Limited Reporting: Hidden Instability in Osaka’s High School Admissions

Osaka’s public high school admissions system uses a common score-based priority order and schools’ preferences, while restricting each student to reporting a single school. We show that the Osaka mechanism is neither priority-stable nor preference-stable, is not strategy-proof for students, and can conceal substantial instability in the admission records. Allowing students to report more schools weakly reduces underlying instability and the number of manipulable students, while making more of the remaining blocking pairs visible. Simulations show that tight reporting compresses quantitative differences across mechanisms. In the unstructured environment, the Osaka mechanism occupies an intermediate position between serial dictatorship and deferred acceptance, but this position disappears in the structured environment. The results show the benefits of broader reporting and the limits of Osaka’s compromise between score-based priority and school discretion.