The United Kingdom might not meet a global education target because of boys' underachievement, the UN says.
Girls getting their GCSE results - they beat boys
The warning is in a report from the United Nations educational organisation, Unesco, about "gender parity" - equal enrolment in school.
It says almost two-thirds of countries have managed this at secondary level.
"In others, significant disparities remain at the expense of boys and have sometimes even widened, as in Sweden and the United Kingdom."
Others that might not meet that target by 2015 include Ireland, Switzerland, Denmark and New Zealand.
The UK has one of the highest disparity figures.
The report says the Department for Education and Skills blames the problem in England on factors including "laddish behaviour, bravado and noise, as boys seek to define their masculinity".
The director of the independent team that drew up the report, Christopher Colclough, said girls moved ahead of boys in reading ability as early as the age of seven or eight.
At GCSE level - when they were 16 - for every 100 girls that got six passes, only 80 boys did so.
Young women maintained the lead at advanced level.
More boys left education when it was no longer compulsory, at 16, which was why the UK might not meet the parity target.
"We've been getting worse over the last decade. That's why the UK is in among the countries in that box," he said.
"It tends to be that girls work better in groups, they mature earlier than boys, have more seriousness about their studies than boys.
"If boys see that they are doing less well, they tend to stop trying rather than be seen as a failure."
The education inspectorate, Ofsted, has also suggested that girls manage to cope better with indifferent or poor teaching - whereas boys needed a greater input.
England's School Standards Minister, David Miliband, said the report drew on data that was two or three years old.
This year, boys had started to narrow the gap at age 14 in English and the government was "committed to cracking the lad culture that stops too many young boys doing well".
"This culture tells boys that it is fine to play around and not work hard, but this harms their chances of fulfilling their potential," he said.
Unless young people, especially boys, were "engaged in their learning" they would continue to fall behind - hence the strategy focusing on improving schooling in the early secondary years.