Against the combined backdrop of population decline, low fertility, and rising mobility, the main challenge in compulsory education is shifting from aggregate shortage to temporal, spatial, and structural mismatch. Taking birth cohorts as its starting point, this article traces a sequence of cohort change, institutional transition, spatial redistribution, and rigidity amplification. Cohort differences generate demand fluctuations; entry ages and school structures create lags between and within stages; migration and school choice relocate demand; and rigidities in facilities, staffing, authorized posts, and finance magnify mismatch. The article therefore calls for continuous cohort monitoring, coordination across stages, school planning based on the population actually served, and more flexible arrangements for places, facilities, teachers, and funding. These measures would strengthen the responsiveness, efficiency, and equity of compulsory education provision.