More than half of Afghanistan's children are not going to school because of a shortage of places and teachers, the aid agency Oxfam says.
There are still far fewer girls than boys going to school
Despite a five-fold increase in school enrolments since the Taleban were ousted in 2001, the education system simply cannot cope, the charity said.
About seven million children are out of school, with girls badly affected.
The report urged rich countries to invest some $800m (£419m) to rebuild Afghan schools in the next five years.
The BBC's Mark Dummett in Kabul says that today there are so many pupils going to school in Afghanistan that a lot of them have to have lessons outdoors.
Others make do in makeshift structures like tents while they await proper buildings.
Oxfam says there are not enough classrooms, books or desks. Teachers, especially women teachers, are in short supply.
Our correspondent says that pay is so low at about $50 a month at best that well-qualified staff prefer other work if they can get it.
Under the Taleban, Afghanistan's girls could only attend classes in secret and there are still many fewer girls than boys going to school.
"Girls are particularly losing out with just one in five girls in primary education and one in 20 going to secondary school," the Oxfam report said.
But Education Minister Hanif Atmar said the situation was not as bad as Oxfam had described.
He told the BBC that his ministry did need much more money from international donors to meet the goal of educating every child for free but he said the government was working hard to increase the number of school places and improve the quality of teaching.
"The enrolment that we have in our schools today, at around six million children, we've never had in our history, so that's a great progress made.
"However, there are still challenges that need to be addressed. The critical issue is training of teachers, in particular female teachers, but for training we do need resources that we do not have adequately at the moment."
Oxfam's report was released on the eve of a summit of Nato leaders in Latvia.
The alliance has a leading role in trying to bring security to Afghanistan.
The country has been promised billions of dollars by the international community for rebuilding, but Oxfam said too little was going on education.
"Rich countries are not providing nearly enough aid to Afghanistan despite their many promises. So far they give only $126m a year," it said.