New figures show the UN has fallen well short of its target of getting as many girls educated as boys.
Outdated sexual stereotypes are holding girls back
The UN children's fund, Unicef, says that out of the 115m children around the world not getting any primary school education, 90m are girls.
At a summit five years ago, world leaders pledged to eradicate this educational disparity by 2005.
Outdated sexual stereotypes, poverty, HIV/Aids and armed conflict are blamed for the failure to achieve this goal.
Unicef says it is a tragedy that so many children, especially girls, have been abandoned to a bleak future.
Many girls are denied schooling because cultural traditions define a female's place as in the home, and social pressure is exerted for them to marry early, sometimes as young as the age of 10, the UN says.
"Male privilege and entitlement (ensure) that when educational opportunities are limited, boys will take available classroom space," the study says.
Forty-six countries will miss the Millennium Goal target to get as many girls enrolled as boys.
According to Unicef's 100-page report, the overall enrolment of children is "unacceptably low".
"Education of children, especially girls, is the cornerstone to national progress," Unicef Executive Director Ann Veneman said in a statement.
"It leads to greater economic productivity, reduced infant and maternal mortality, and a greater likelihood that the next generation of children will go to school," she added.
Many of the countries highlighted in the report are in sub-Saharan Africa, but Turkey, which recently began accession talks to the EU, was also listed.
The BBC's Sarah Rainsford in Istanbul says that two key things still keep many girls out of school in Turkey - poverty and tradition.
In theory, both sexes have had equal access to education since the Turkish republic was founded, but in practice boys still tend to be favoured, our correspondent says.
The problem is worst in the rural east and south-east of Turkey, and poor areas of the big cities, where children are often used for seasonal work.
A high-profile campaign by Unicef backed by the Turkish government has helped to close the gap to just under 6% on average.
In some areas, volunteers have been going door-to-door encouraging families to give their daughters an education.
But more than half a million Turkish girls are still kept out of school.