UNICEF Executive Director Carol Bellamy said investment in education was the best way to close the gender gap.
"We believe that the failure to invest in girls' education puts in jeopardy more development goals than any other single action that could take place," she told The Associated Press.
In its annual State of the World's Children report, UNICEF said 121 million children around the world are out of school - with a larger number of girls, compared to boys, in this situation.
"When a girl is without the knowledge and life skills that school can provide, there are immediate and long-term effects; she is exposed to many more risks than her educated counterparts and the consequences are bequeathed to the next generation," the study said.
U.N. "millennium goals" on poverty reduction commit the world to parity for boys and girls in primary education by 2005, but most specialists acknowledge that this will be impossible to achieve.
"The single largest obstacle to girls going to school is school fees, even though in many places it costs almost as much to collect them as is collected," Bellamy said. "We strongly urge the abolition of school fees."
When poor families are forced to make a choice, they decide to pay for the education of their sons. That doesn't mean they don't want their daughters to be educated as well, she said.
Bellamy gave the example of Kenya, where school attendance has shot up by at least 1.2 million since primary school fees were abolished at the beginning of this year.
Throughout Africa, UNICEF said, a push to get girls into school has seen big improvements. Over five years, school enrollment rates for girls rose by 15 percent in Guinea, 12 percent in Senegal and 9 percent in Benin.
In the most striking example, the number of girls enrolled in first grade in the central African country of Chad quadrupled in two years.
Despite the successes, however, at the current rate of funding it was estimated that it will take until 2129 to achieve universal primary education in sub-Saharan Africa, according to UNICEF.
UNICEF said campaigns to educate girls also benefit boys because it is rare for education to be offered to girls alone, although the reverse might not be true.
The 147-page study said that universal education has widely been considered a luxury rather than a basic human right.
It said countries and donors often have considered that boosting economic performance will lead to social gains like schooling for girls but in fact the reverse is true - improving social welfare leads to economic progress.
But educating girls in particular also has wider social benefits, Bellamy said.
"A girl gets an education and she is more likely to be healthy. Her children are less likely to die before the age of 5. She is more likely to make choices about her life," she said. "It doesn't make it all go away, but she becomes more of a functioning person in society."
Because educated girls and women better understand health issues, every extra year of education reduces the number of women who die in childbirth by two per thousand, the study added.
UNICEF called on politicians and other leaders to make girls' education a core component of development efforts, ensure that primary education is free and universal and hold governments accountable for progress.
It also called for increased international funding for education. UNICEF estimates that donors may need to provide as much as $60 billion between now and 2015 to ensure girls' education, but the agency says education is an "ideal investment" because of the wide benefits.
"Girls' education costs money, but it isn't a break-the-bank price tag item," Bellamy said.
By Naomi Koppel