There has been "nearly zero progress" in tackling the lack of access to school in many of the world's poorest countries, a UN agency has said.
There are still 123 million school-age children without schools, Unicef said.
The UN's children's agency blamed "pervasive levels of poverty", conflict and humanitarian emergencies.
International pledges to create school places were not addressing "the realities of a volatile world", head of education Jo Bourne said.
The warning from Unicef said despite targets by world leaders, there had been little substantial progress in the past decade in ending the problem of children not having schools to attend.
Unicef said efforts to increase the participation rate in school had "stagnated".
The problem is most acute in sub-Saharan Africa, where the lack of school places has been exacerbated by an increasing population.
Those most likely to miss out are the poorest families from the poorest countries.
Conflicts in Syria and across the Middle East have reversed improvements in education levels in the region, with an additional 3.4 million children out of school.
About a fifth of children in the world without access to school are living in conflict zones.
They might be refugees whose education has been interrupted or living in areas where it is too dangerous to go to school or where school buildings have been destroyed.
But despite suggestions of a lost decade in expanding access to education, there have been some improvements.
The proportion of children aged six to 15 without schools was now 11.5%, said Unicef, down from 12.8% a decade ago.
The UN agency highlighted increases in children going to school in Ethiopia and Niger.
There had been a pledge by the international community in 2000 to provide a primary education for all children by 2015 - as part of the millennium development goals, it said.
There had been rapid progress towards this target in the early years of the century, but the financial crisis had seen a reduction in international aid.
The target for universal primary education was missed in 2015. And another UN agency, Unesco, warned that the latest targets, the sustainable development goals, were already likely to be missed.
The international goals promise universal primary education for 2030, but Unesco said that on current trends this would not be achieved until 2042.
The UN has also calculated that another 69 million teachers would need to be recruited to deliver international education promises for 2030.
More from Global education
- If schools had to choose between behaviour and creativity...
- 'Dragon's Den' for migrant entrepreneurs
- Canada 150: How Canada became an education superpower
- Canada 150: Top teacher stands up for indigenous students
- Canada 150: Trump's biggest fans? Canadian universities
- School funding not always linked to results
Ideas for the Global education series? Get in touch.
Ms Bourne, said current attempts to keep up with population growth were not enough.
"This business-as-usual approach will not get the most vulnerable children into school - and help them reach their full potential - if they continue to be trapped in poverty, deprivation and insecurity," she said.
Ms Bourne said that making more rapid progress on getting children into school would depend on tackling the underlying problems - such as ending conflicts that made school unsafe.
There were also shortfalls in funding from international donors.
Unicef said that up to June 2017, it had received only 12% of the funding needed to educate children in areas affected by a humanitarian crisis.
Ms Bourne said there needed to be "greater and more predictable funding for education in unpredictable emergencies".