This study evaluates the impact of Poland’s 2017 education system reform on secondary schools in Warsaw. The reform eliminated middle schools, restored K-8 primary schools, and shortened the duration of common compulsory education from nine to eight years. Using a quasi-experimental design comparing two cohorts transitioning under different systems, we investigate changes in school segregation and student sorting. We found that the reform reduced school segregation, with the share of test score variance attributable to between-school at end of middle/K8 education decreasing from 38% to 25%. However, K-8 graduates entered an equally stratified secondary education system, reflecting persistent inequalities in access to high-quality schools. These results highlight the reform’s mixed success: although it successfully balanced academic achievement levels among 15-year-old students during primary education, selective admission practices continued to sort students by ability at the upper secondary level.