The biggest barrier to girls' education around the world is school fees, a charity argues.
Worldwide girls are less likely than boys to go to school
Save the Children says in a report that 17 of the 25 countries with the most girls not in school still charge fees.
It says another 4.5 million children would go to school if fees were abolished in 13 countries in sub-Saharan Africa, for instance.
The report focuses on girls, who are said to make up 60% of the 100 million children worldwide not being schooled.
Save the Children says that in Liberia, for example, sending one child to school costs half the average income of £62.
In UK terms this would be the equivalent of a family on an average income of £17,000 spending £8,500 a year for one child's education.
When Uganda scrapped fees in 1997, the number of girls in school more than doubled to three million within three years, it says.
But many countries cannot afford to do this, says the report, which has been published in advance of next week's UN summit.
So it calls on world leaders to make a commitment to achieving the education Millennium Development Goals of ensuring all girls and boys complete a primary education by 2015.
Save the Children's director of policy and communications, Fiona Weir, said: "Giving girls an education is one of the best ways to reduce poverty, malnutrition and HIV/AIDS but unless world leaders act now, they will condemn yet another generation to poverty.
"The UN Summit will be make or break. World leaders must show real commitment to abolishing school fees as a crucial step to making this a reality."