On average, primary school students in OECD countries receive 805 hours of instruction per year, and lower secondary students 916 hours, spread over 38 school weeks. However, these averages mask wide variations across countries. The total length of school vacations averages around 14 weeks per year, ranging from less than 11 weeks in Costa Rica and Denmark to 17 weeks in Greece, Latvia and Lithuania. The organisation of the school year, in particular the length of the summer holidays, is frequently debated but is rarely the subject of educational reforms because of its sensitive nature. Contrary to common assumptions, the length of instruction time is not closely related to students’ academic performance. The quality of instruction and other factors such as students’ participation to private tutoring and extracurricular educational activities, play critical roles in determining learning outcomes, too.