Fewer school days, more inequality

•Access the impact of school-day reduction on study time use and academic achievement.•Japan Time Use Survey and PISA are the data sources.•The socioeconomic gradient of 9th graders’ study time becomes 110% steeper.•The socioeconomic gradient of 10th graders’ reading test scores becomes 20% steeper.•Intensive compulsory education reduces the inequality of academic achievement.

This paper examines how the intensity of compulsory education affects the time use and academic achievement of children from different socioeconomic backgrounds. The impact is identified off the school-day reduction of Japan in 2002 that resulted when all Saturdays were set as public-school holidays. An analysis of time diaries and test scores reveals that the socioeconomic gradient of 9th graders’ study time becomes 110% steeper and the socioeconomic gradient of 10th graders’ reading test scores becomes 20% steeper after the school-day reduction. Intensive compulsory education contributes to equalizing the academic performance of children from different socioeconomic backgrounds, at least for some subjects.