A new report is drawing attention to the plight of under-educated girls as the U.N. marks International Day of the Girl Child on Thursday.
Plan International says while school enrollment levels for girls and boys have reached "global parity" at the primary school level, school completion rates for girls have lagged behind.
The child development organization says 39 million girls between the ages of 11 and 15 are out of school.
Plan International CEO Nigel Chapman says a number of factors often prevent adolescent girls from remaining in school.
"As girls get older, they are more likely to be asked by their families to contribute economically, either through being part of the family business, working in the fields and so on. And that is, of course, a pull away from attending school every day," he said. "It is a short-sighted pull in many ways because it means, in the end, those girls don't have the skills they need to get the jobs they ought to have in the future."
He says the problem of adolescent girls not attending school is most acute in Africa.
Plan International says other problems that contribute to their drop in enrollment include early pregnancies and the cost of school supplies and uniforms.
Also, the group says some girls experience violence at school - a fact highlighted in Pakistan this week when an alleged Taliban gunman shot and gravely wounded a 14-year-old girl who had spoken out for the education of women.
During a Wednesday appearance marking "International Day of the Girl Child," U.S. Secretary of State Hillary Clinton said limited education poses lifelong problems for women.
"Every year, 10 million girls under the age of 18 become child brides and many of them [are] under the age of 16, and many of those girls are forced into early marriage which robs them of the opportunity to continue their education and it threatens their health and it traps them in lives of poverty," she said.
In a report marking the International Day of the Girl Child, the U.N. called early marriages a "fundamental violation of human rights" that "impacts all aspects of a girl's life."
The world body noted that in Afghanistan, saying 46 percent of Afghan women were married before the age of 18, and more than 15 percent before age 15.
Some information for this report was provided by AP, AFP and Reuters.