One of the most obvious and not sufficiently well understood political decisions in education regards the optimal amount of instruction time required to improve academic performance. This paper considers an unexpected, exogenous regulatory change that reduced the school calendar of non-fee-paying schools (public and charter schools) in the Madrid region (Spain) by two weeks during the 2017/2018 school year. Using difference-in-differences regression, we found that this regulatory change contributed to a significant deterioration in academic performance, particularly in Spanish and English. We further explored non-linear (quantile) effects across the distribution of scores in standardized exams, finding that the disruption due to the new regulations affected more students in the upper quartile of the distribution. Overall, we found a reduction in the gap across non-fee-paying schools and an increase in the gap between non-fee- and fee-paying schools (private schools).