Background: Despite much discussion on the role of education policy on school and student performance, we know little about the effects of school spending at the margin on student cognitive achievement beyond the effects of class size. Purpose: The paper examines the effects of annual ninth grade classroom hours in literacy and maths on ninth grade (aged 16) student performance in writing and maths, respectively. Programme description: In Denmark, primary school consists of the first to the ninth grades. Before 2003, only few national regulations governed classroom hour administration in public primary schools, resulting in large variations in the number of classroom hours across the country. Thus, following national discussions about improving skill formation in a heterogeneous student body, in July 2003 the Danish Ministry of Education made changes to classroom hour planning that immediately reduced variation in classroom hours across schools. Sample: I use a sample of 64% of all ninth grade students (aged 16) in Denmark in 2003-2006, equivalent to 144,739 students and 921 schools (58%) for literacy and 144,618 students and 924 schools (58%) for maths. Using data from administrative records from various registers and through unique personal and institution identifiers, I first link school characteristics to the students and, second, link students to a long list of individual and parental background characteristics. Design and Methods: The paper exploits the unique policy-induced variation in classroom hours in a one-year period before and a three-year period after the reform. As Danish municipalities are the local school authorities and as the reform narrowed the gap in classroom hours across them, the municipalities experienced differences in intensity to treatment. Thus I use a school fixed-effects model, where an interaction-term between the continuous treatment -- classroom hours -- and the year of policy implementation define the effects of classroom hours in literacy and maths on student achievement in writing and maths, respectively. Results: On average, the reform changed classroom hours by 2.2-3.3% in literacy and maths, with an impact on student achievement. For literacy I find no significant effects of classroom hours, but for maths I find stronger effects. One additional hour per year increases the maths score by 0.21% of a standard deviation, decreases the probability of obtaining a test score below the mean by 0.01%, and increases the probability of obtaining a test score above the mean by 0.08%. One possible explanation for this difference between subjects is that training in literacy takes place in the home environment more than maths and thus is less sensitive to classroom hour changes. Conclusions: The findings are considerably important when placed in the context of debates about intra-school resource allocation. In maths, classroom hours changed on average only by 2.63 annual hours from 2003 to 2004. However, I still find effects of these generally small changes to classroom hours; thus student achievements are sensitive to even small changes in classroom hours.