The more homework that students have, the better their schools do, figures from the latest Programme for International Student Assessment (Pisa) show.
But the same does not necessarily apply at country level, owing to the wide variation in the types of extra learning each nation encourages. Not all out-of-school work appears to be equal.
The Organisation for Economic Cooperation and Development study measures the time that 15-year-olds spend studying in school, at home or with tutors. On average, students who spend more time learning have higher grades, but it is harder to determine how best to organise after-school lessons and homework to improve students' skills.
In richer nations, students report spending an average of 4 hours 53 minutes on homework per week, of which 1 hour 18 minutes is supervised. Time with a tutor is on average 39 minutes, with 37 minutes spent in after-school classes and an hour with family members.
Schools set different amounts of work, but in 59 of the 65 countries measured students at institutions in richer areas did on average more homework than those in poorer ones.
Students attending after-school lessons - OECD average2
27% of students attend after-school lessons in the language of instruction
26% of students attend after-school lessons in science
38% of students attend after-school lessons in mathematics
Source: Pisa 2012 Results: What Makes Schools Successful? Resources, Policies and Practices, chapter 3, Excel tables part 1, 1 table IV.3.27; 2 table IV.3.25, Organisation for Economic Cooperation and Development. bit.lyKeyFindings. 3 Source: Pisa 2012 Results: What Students Know and Can Do, chapter 2, figure I.2.13, OECD. bit.lyPisa2012Maths.