The number of school-age migrant and refugee children has risen 26 percent since 2000 to some 18 million, the UNESCO cultural agency wrote in its 2019 Global Education Monitoring report, AFP reported.
Around half the world's forcibly displaced people are under 18, and often have little if any access to public education systems in the countries where they are seeking asylum.
Even if they are not excluded outright, host countries often lack the resources to offer language classes and ensure the integration of refugee children.
Lebanon and Jordan, with the largest number of refugees per capita as people fled the civil war in neighboring Syria, have imposed separate morning and afternoon classes for citizens and refugees.
Even wealthy Germany would need 42,000 new teachers to properly educate the refugee children taken in as part of Chancellor Angela Merkel's open-door policy enacted in 2015, which has since been sharply curtailed, the report found.
"Education is the key to inclusion and cohesion," UNESCO's chief, Audrey Azoulay, said in a statement.
"Increased classroom diversity, while challenging for teachers, can also enhance respect for diversity and be an opportunity to learn from others," she said.
The report also found that first-generation migrants represented 18 percent of students in high-income countries in 2017, up from 15 percent in 2005, representing 36 million students.
But in the EU these students are twice as likely to drop out of the education system early as native-born students.
And in the 34 countries of the Organization for Economic Cooperation and Development (OECD) they were 32 percent less likely on average to achieve basic skills in reading, math and science in 2015.
"Countries cannot think the job is done once immigrants are in school," said Manos Antoninis, the director of the education report.
Too often, he said, "They end up in slower school tracks or in under-resourced establishments in troubled neighborhoods."
UNESCO said that in the two years since the adoption of the New York Declaration for Refugees and Migrants in 2016, refugees have missed a combined 1.5 billion days of school.