This paper examines the full-time school program in Rio de Janeiro's municipal schools. The program, called as the "Single-Shift" schools (Turno Unico), extends the time students spend in municipal schools and seeks to improve the quality of education provision by creating a diverse curriculum for the use of the extra time in school. Unlike the model prevalent in most Brazilian public schools, in which the school day is divided in two shifts of four to five hours each, Single-Shift schools provide education in a format in which students attend a seven-hour daily shift. A subset of Single-Shift schools was certified when they included aspects such as having all teaching staff fully dedicated to a single school. Difference-in-differences estimates, including school and time fixed effects, as well as restrictive school-by-time controls, indicate sizable and robust positive results for the certified Single-Shift program in middle schools. The results indicate that just extending the school day does not grant positive impacts on student performance if it is not also coupled with a more comprehensive and careful consideration on how the additional school hours are used and organized, which requires a well-structured and integrated curriculum, teachers fully dedicated to one school, and focused teacher training.