Making class sizes smaller sounds like a success story.
But an international analysis of its impact shows unintended consequences - it often seems to mean lower pay for teachers and there isn't much evidence that it brings better results.
Reducing class sizes has been a popular policy in many countries, often supported by parents, politicians and teachers.
It has been one of the big trends of the last decade.
Class sizes fell on average by 6% between 2006 and 2014 in the lower secondary school years in member countries of the Organisation for Economic Co-operation and Development (OECD).
This includes more than 30 of the most developed countries, including most of western Europe, Japan, Australia and the United States.