Anti-corruption campaigners Transparency International have published a global survey showing that about one in six students has had to pay a bribe for education services.
In parts of sub-Saharan Africa and Asia this might be requiring parents to pay a fee for a school place that should be free.
In Eastern Europe, it might be paying to gain an advantage in university admissions.
The Berlin-based campaign group is best known for its annual "global corruption barometer", which measures levels of dishonest payments in more than 100 countries, based on more than 114,000 household interviews.
This year's survey also asked questions about people's first-hand experiences and perceptions of dishonesty in education.