Education in Brazil and Latin America is typically associated with a shorter school day than in most OECD countries. This paper assess the impact of a Brazilian policy that extends the high school day on student test scores. The program lengthens the school day while adding resources for participating schools. The number of instructional hours for math and language classes increased by 50% and 20%, respectively. Our estimates show that over three years of high school (10th to 12th grades), the full-day school program increased math and language student test scores by 0.22 and 0.19 standard deviations respectively. Our results are robust to several specification checks that aim to mitigate potential selection bias concerns.