This article reports the results of a study in which the basic regression-discontinuity approach to assess the effect of 1 year of schooling is extended. The data analysis covers the 6 grades of secondary education in Cyprus and thus assesses the contribution of secondary education to the cognitive development of 12- to 18-year-old students. A model with multiple cut-off points is fitted. Achievement scores on both curriculum-based tests in language and mathematics and the non-curriculum-referenced Comprehensive Test of Cognitive Development (CTCD), which measures the development of 5 domains of reasoning (i.e., categorical, quantitative, spatial, causal, and propositional), were analysed to measure the effect of schooling. Implications of findings for the development of educational effectiveness research are drawn.