They could fill half a million classrooms - and even those in school often aren't getting a quality education or have trained teachers.
The enormous challenge of providing quality education to all the refugee and migrant children in the world is revealed today.
The number has grown by 26% since 2000 and they could now fill half a million classrooms, according to a new United Nations report that calls for greater efforts to integrate displaced children.
Half of all the world's forcibly displaced people are under 18 and many have little or no access to public education systems. In the two years since the landmark New York Declaration for Refugees and Migrants in 2016, refugees have missed 1.5 billion days of school.
For those who are in school, their host countries often lack the resources to offer language classes and ensure the integration of refugee children.
The global picture is painted in UNESCO’s 2019 Global Education Monitoring Report (GEM): Migration, displacement and education, which was released today on Universal Children's Day.
"Education is the key to inclusion and cohesion," said UNESCO Director-General Audrey Azoulay.
"Increased classroom diversity, while challenging for teachers, can also enhance respect for diversity and be an opportunity to learn from others."
Those thoughts were echoed by Angela Monsalve, principal of Presbitero Luis Rodolfo Gomez Ramirez school in Colombia. She said: "The biggest challenge for refugee children when they get into our school system is to adapt to a school world that is very different."
But there are signs of improvement, said UNESCO. Eight of the 10 countries hosting the largest refugee populations - such as Chad, Ethiopia and Uganda - include them in their national education systems.
The GEM Report highlighted where countries are succeeding and falling short in ensuring the right of migrant and refugee children to benefit from quality education. These include:
Kenya allows refugees to benefit from the national education curriculum. But its refugee learners are living in camps where they are unable to interact with children from the host communities.
Lebanon and Jordan host the largest number of refugees per head of population but do not have the resources to build more schools. They have set up "double-shift" schools for host and refugee children - but UNESCO says that limits interaction between the two groups.
Rwanda and Iran has invested heavily to ensure refugees attend school alongside nationals.
Turkey has committed to include all refugees in its national education system by 2020, as have seven countries in East Africa.
But UNESCO says these efforts will succeed only if there are enough trained teachers. In Lebanon, just 55% of teachers and staff received specialised training to meet the needs of displaced learners in the past two years.
To provide quality education to all refugees, Germany would need 42,000 new teachers, Turkey 80,000 and Uganda 7000.
“Countries cannot think the job is done once immigrants are in school," said Manos Antoninis, Director of the GEM Report.
"They are being excluded in so many other ways. They end up in slower school tracks or in under-resourced establishments in troubled neighbourhoods".
But he said almost all countries are signing up to global agreements on refugees and migrants that include key education commitments. He added: "This could be the much-awaited turning point.”
The report shows that sub-Saharan Africa continues to bear a large part of the refugee crisis. It houses almost a third of all refugees in the world along with millions of internally displaced people, which is putting huge strains on already struggling education systems
Low and middle-income countries host 89% of refugees - but they lack funds to cope. UNESCO said donors need to triple the $800 million spent on refugee education in 2016 and ensure long-term support.
It said the Education Cannot Wait fund - set up in 2016 to provide education in emergencies after campaigning by Theirworld and others - is an example of global efforts to bridge humanitarian and development aid and finance refugee education.
At the launch of the GEM Report in Berlin today, Germany announced a new contribution of about $17 million to Education Cannot Wait, making it the fund's third largest donor.