Girls still face "sharp discrimination" in getting access to schooling in most developing countries, according to a global survey of education trends.
A lack of female role models is said to be an issue in India
A new report from the United Nations educational organisation, Unesco, talks of "significant" recent progress.
But equal enrolment remains "a distant prospect" in 54 out of 120 countries.
"While not a complete surprise, these results are obviously a cause for deep concern," said Unesco's director-general, Koichiro Matsuura.
In some countries - the UK being prominent - the report shows an imbalance the other way, with more boys failing to complete secondary education, the report says.
But in the most populous country, China, it says boys will continue to outnumber girls in secondary schools for many years to come.
"Gender parity in education is a priority not only because inequality is a major infringement of fundamental human rights but because it represents an important obstacle to social and economic development," said Mr Matsuura.
The director of the independent team which prepared the report, Christopher Colclough, said investing in girls' education had "a high pay-off".
He said: "Education helps to increase their productivity to a significant extent, thereby adding to household incomes and reducing poverty."
When parents, especially mothers, were educated, their children tended to be healthier and had a greater chance of doing well at school.
"Investing in educating girls now is one of the best ways of ensuring that future generations will be educated," he said.
The report says that in the 1990s, the number of girls in primary school increased faster than that of boys.
Countries where girls have least access to primary school
It says 57% of the estimated 104 million primary-age children out of school worldwide are girls.
Of 128 countries, 52 have already achieved gender parity or will have done so by the 2005 target date.
The report uses what is known as the "gender parity index", where a figure of one represents parity between the sexes.
Various factors are identified as working against girls' greater participation.
The need to help provide for the family was a key factor - with many more girls than boys being involved in domestic labour.
In many countries cost is a factor, even though there is supposed to be free primary education for all.
Also, in some places girls marry young - 40% by the age of 15 in Nepal, for example.
Infection with HIV/Aids is much higher among girls than boys in Southern Africa and the Caribbean, "a disparity linked to widespread exploitation, sexual abuse and discriminatory practices," the report says.
Often there are no female role models among the teaching staff.
When it comes to secondary schooling, however, girls predominate in several countries. The "parity index" is 1.05 in Bangladesh, Denmark and Mexico.
It is even higher elsewhere: New Zealand (1.06), Bahrain, Iceland, Russian Federation and Trinidad and Tobago (1.07) Colombia and the Philippines (1.10), Malaysia (1.11), United Arab Emirates (1.12), United Kingdom (1.17), Suriname (1.18) and Sweden (1.26).
In England, the School Standards Minister, David Miliband, said: "Underachievement by boys is a long-term problem in schools but we are 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."
Gender parity by 2005 is one of the six goals of the Education For All programme endorsed by 164 governments at the World Education Forum in Senegal in April 2000.
But parity does not mean equality, the Unesco report says.
In many countries - notably in Europe, Latin America and Asia - boys do less well educationally but do not lag behind economically or politically.
So women may have to work harder and do better than men, it says, "if they are to be successful in competition for jobs, equal pay and decision-making positions."
The target date for gender equality is 2015.
Read a selection of your comments below.
I sympathise with Unesco's ideals of parity and equality to reduce poverty. But I also wonder if this is a Western view, and whether it should be imposed as a goal for all nations regardless of local culture. Surely each society should be allowed to organise its own social structure? I don't buy this one-size-fits-all mentality where a few unelected UN bureaucrats decide how everyone else should live - a new dictatorship.
Simon Bisset, UK
I sponsor children in the Third World and was shocked to learn that often girls are not sent to school. The sisters of my sponsor child in Mali now attend, but they are privileged to do so. Better awareness is needed for women to realise they are valued and can achieve.
Flash Wilson, UK
The key statement above is that where boys do less well educationally they still do better economically and politically. This means that while the push for educational equality is vital, the wider issue of discrimination against women is still not being addressed.
Miss Susan Darby, UK