This paper investigates the effect on academic performance of an exogenous educational reform that reduced the school calendar of non-fee-paying schools in the Madrid region (Spain) by approximately two weeks, leaving the basic curriculum unchanged. To identify the consequences of such a measure, we exploit the fact that it did not affect private schools (control group) and the existence of an external cognitive test that measures academic performance before and after its application in the region. We find that the reform worsened students' educational outcomes by around 0.13 of a standard deviation. This effect was especially strong in the subjects of Spanish and Mathematics. We further explored quantile effects across the distribution of exam scores, finding that the disruption had a more negative effect on students in the upper quartile than those in the lower quartile. Overall, the analysis shows a reduction in the gap across non-fee-paying schools and an increase in the gap between non-fee- and fee-paying schools