This blog was first published in the Huffington Post.

In the midst of the opening of the United Nations General Assembly in New York, it is encouraging to reflect on one of the most important development priorities to emerge in the past 15 years -- girls' education.

It is now well recognized that educating children, and particularly girls, is central to the economic, social and political development of a country. The link between increased access to quality basic education and improved maternal and child mortality, nutrition and food security, economic growth, sustainability and social stability is now evident to most leaders.

There is progress, and yet so much more to be done

This knowledge is now so central to development planning that in most countries more girls than ever go to school today. The number of girls not in school has dropped by 47 percent over the last decade.

But even though progress is being made, millions of girls are still missing out.

Around 31 million girls today don't go to primary school, making up over half of the world's 58 million out-of-school children. These girls are among the hardest to reach children in the world: they often live in very poor, marginalized and remote communities or in places affected by war and instability.

Drilling a little deeper, we see that inequality and discrimination are also major reasons why girls are disadvantaged in education -- not only in access to school, but also in learning outcomes.

Turning gender policies into reality isn't easy

Despite the rising school enrollment rate of girls, far fewer girls than boys complete primary school, and they are less likely to make the transition from primary to secondary school.

In Tanzania, for example, 85 percent of girls and 77 percent of boys complete primary school, and yet less girls than boys finish secondary school: only 22.5 percent of girls compared to 28 percent of boys. In Pakistan, only 42 percent of women can read and write compared to 67 percent of men. Throughout school life, girls are more likely to suffer discrimination and violence, with higher dropout rates than boys.

In many countries, attitudes about girls' education are changing, and government policies are actively supporting gender parity. But turning policies into reality is hard, and requires social as well as policy change. Let us not forget the hundreds of abducted schoolgirls who are still missing in northern Nigeria. As Malala Yousafzai reminds us with her eloquence and determination, going to school is a high-risk choice for millions of girls around the globe.

Equity means being fair and impartial

An important lesson from the Millennium Development Goals (MDGs) is that the promise of education cannot be fulfilled without equity. We already know that we will not achieve MDG 2 -- universal primary education -- by 2015. This is not only because policies and minds need to change, but because resources have not always been directed to those that need them most -- to girls' education, disabled children, those living in conflict, or in remote, marginalized communities or just on the bottom of society's pyramid.

It is these hardest-to-reach children that the Global Partnership for Education seeks to reach. We believe this is the only way that we will ever attain our global development goals and build a more just, stable, healthy and sustainable world for us all.

As the United Nations Member States embark on the next round of negotiations on the post-2015 development framework, a stand-alone education goal must be a central pillar -- and I am sure it will be. But that's not enough. It will be essential that the goal has equity at its heart which mean fair and impartial access to school for all children and that adequate resources are made available to attain it.

No progress without sufficient funding

The recent GPE Replenishment Conference secured much-needed funding -- US$ 28.5 billion for 2015-2018. Donors are again stepping up after years of decline in aid for education -- but even more importantly, our developing country partners have made outstanding commitments to increase their domestic financing for education. Many developing countries pledged to increase their education spending to 20 percent of national budgets within four years. That's a great success and shows the importance that education holds for so many countries.

Better planning needs better data

Access to data will be critical in monitoring education progress, and this is one of our toughest challenges. Education data is woefully lacking in developing countries making evidence-based decision-making all but impossible. Recognizing the needs of the most disadvantaged can be hard to spot.

As girls are critical to successful education outcomes -- we especially need to ensure we collect gender-sensitive, disaggregated data. Girls' access to schooling, their progress through school and learning outcomes will tell us a lot about what works, and what doesn't. Family and social factors should also inform policy-making -- we need better data on early marriage rates, age of first pregnancy, girls' nutrition and distance from schools.

With such information, governments can make sound decisions such as hiring more female teachers. Building schools closer to where girls live has reduced travel time to schools and improved girls' enrollment in Yemen and Afghanistan, two GPE partners countries. In Africa, ensuring girls have access to adequate toilets, and schools are safe from violence and sexual harassment, has seen girls' attendance and learning outcomes rise.

As we close in on 2015, let's celebrate progress -- there are many millions more children in school compared to 2000. We won't achieve our goal if we don't focus on greater equity and more resources for the least advantaged children. Let's make sure we count girls, and count on girls.