The UN children's fund, Unicef, says in a new report that the hopes of a generation across the Middle East and North Africa are being shattered.
It says that in Syria, Iraq, Yemen and Libya, almost 9,000 schools are not able to be used for education.
Unicef also documented scores of attacks on schools and teachers across the region.
Peter Salama, regional director for Unicef in the Middle East and North Africa, said: "The destructive impact of conflict is being felt by children right across the region.
"It's not just the physical damage being done to schools, but the despair felt by a generation of schoolchildren who see their hopes and futures shattered."
The 13.7 million who are out of education represents about 40% of the school-age children in Syria, Iraq, Yemen, Libya and Sudan and the UN fears that figure could top 50% in the coming months.
Unicef says that, in 2014, there were 214 attacks on schools in Syria, Iraq, Libya, the Palestinian territories, Sudan and Yemen.
One in four schools in Syria have shut since March 2011, affecting more than two million children, it says.
Unicef's Education Under Fire report added that "the killing, abduction and arbitrary arrest of students, teachers and education personnel have become commonplace" in the region.
Thousands of teachers have abandoned their posts in fear and children are often forced to work illegally to help their families.
The children can also become combatants at a young age.
Mr Salama said Unicef needed an additional $300m this year to try to improve access to education in the region.

Downie, 15, became the first British woman to win an international all-around medal in her first senior year.
Purvis also finished third in the men's competition to claim his second career European all-around bronze.
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Fellow Britons Sam Oldham and Claudia Fragapane finished sixth in their respective overall competitions.
Downie, in her first senior year, scored 56.623, behind winner Giulia Steingruber of Switzerland and Russia's Maria Kharenkova.
"I'm over the moon. I never thought coming here I'd be among the medals," Downie told BBC Sport in Montpellier. "I was hoping for a top six or eight finish - it's just absolutely amazing."
Downie, younger sister of European uneven bars champion Becky, made a good start on bars then followed with solid routines on beam and floor - where she was marked down for stepping out of the field.
She finished strongly with 14.8 on the vault - her strongest apparatus, and the one where she will aim for another medal in Saturday's final, as well as the uneven bars.
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"I was really nervous going into the vault," she said.
"I knew if I nailed it I could have a shot at a medal so I just tried to keep calm and do what I do in training."
Britain's second medal of the day came thanks to 24-year-old Purvis who scored 87.423 over his six apparatus routines.
Ukrainie's Oleg Verniaiev took the all-around gold ahead of David Belyavskiy of Russia.
The UN has said that conflicts in the Middle East are depriving more than 13 million children of an education.
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Ellie Downie and Dan Purvis each delivered Great Britain a bronze medal at the European Gymnastics Championships in Montpellier.