The Finnish comprehensive school system is regularly referred to as a uniform and "no-tracking". In this article, we show with novel urban case data in Finland that school performance differed significantly between schools, most strikingly between school classes, and was connected to the school's selectiveness in pupil admission. A comprehensive registry data of 13-year-olds in all schools and school classes of one Finnish city was used. Via the case of urban Finland, it is discussed how school choice policies, and thus pupil selection, has led to hidden ability grouping although applied among totally publicly run basic schooling. Thus the traditional perception of a uniform Finnish comprehensive schooling seems not to be valid in the urban contexts of our study.