Warfare and conflict are preventing 25 million young people from getting any access to school, says Unicef.
The United Nations children’s agency warns of the long-term cost of political violence, particularly in parts of sub-Saharan Africa.
In south Sudan, Unicef says almost three-quarters of primary-school-age children are missing out on education.
Unicef’s Josephine Bourne says this threatens the “stability of families, communities and economies”.
In conflict zones in 22 countries, about one in five school-age children is missing out on education, says Unicef.
There have been warnings that without any chance of school, young people will be condemned to a cycle of poor employment, poverty and risk of exploitation and recruitment into violence.
Among the worst-affected countries is Chad, where about half of children are unable to get to school, with girls particularly likely to miss out.
Highlighting the global scale of the challenge, Unicef brought an education campaigner to Chad who was herself a refugee from the civil war in Syria.
Muzoon Almellehan, 19, visited education projects in Chad trying to support families who had fled from violence by Boko Haram insurgents in Nigeria.
Among child refugees arriving in Chad from Nigeria, Unicef says 90% have never been in school.
“Meeting children in Chad who had fled Boko Haram reminded me of my own experiences in Syria,” said Ms Almellehan.
“Education gave me the strength to carry on.
“I wouldn’t be here without it.
“Conflict can take away your friends, your family, your livelihood, your home.
“It can try to strip you of your dignity, identity, pride and hope.
“But it can never take away your knowledge.”
In Chad, Ms Almellehan met a Nigerian girl who had been abducted by Boko Haram and then “drugged, exploited and abused” for three years before escaping.
The Islamist militant group is still holding about 195 of the schoolgirls abducted three years ago from Chibok in north-eastern Nigeria.
One of the schoolgirls who escaped, speaking last month, called on the international community not to forget those still in captivity.
The Chibok student, speaking on condition of anonymity, said: “These girls are human beings, not something that we can forget about.”
But Unicef has warned its work in supporting education is struggling with inadequate funding.
The UN agency says in Chad it has only 40% of the budget needed for this year’s education work.
Conflict and violence have been significant barriers to the goal of providing a primary school place for all children.
There have also warnings about inequality within developing countries – where wealthy families have access to education, but poor, rural families are excluded.
The international community set targets for universal primary education by first 2000 and then 2015, which, despite some progress being made, were both missed.
The current target, part of the sustainable development goals, aims for this to be fully achieved by 2030.
Although Unesco, the UN agency that monitors global access to education, warned last autumn that, on current trends, the target was already unlikely to be achieved.