Abstract: We evaluate the effects of the school system on mathematical, verbal, and logical reasoning skills using data from the Finnish comprehensive school reform that abolished the two-track school system. We use a difference-in-differences approach that exploits the gradual implementation across the country. Cognitive skills are measured using test scores from the Finnish Army Basic Skills Test. The reform had small positive effects on verbal test scores but no effect on the mean performance in the arithmetic or logical reasoning tests. However, the reform significantly improved the scores of the students whose parents had less than a high school education.