Around 78.2 million of the children are estimated to be completely out of school due to often protracted conflicts and emergencies, UN says in a new report.
The number of children forced out of school or who have seen their education disrupted in conflict- and crisis-torn countries has nearly tripled in six years to 222 million, the UN said.
That is up from 75 million children estimated to be in the same situation in 2016, the United Nations' Education Cannot Wait (ECW) programme found in a fresh report on Tuesday.
This is a "shocking, shameful number," ECW chief Yasmina Sherif told reporters, stressing that "222 million dreams" were being dashed by lacking access to uninterrupted education.
A full 78.2 million of the children are estimated to be completely out of school due to often protracted conflicts and emergencies - 54 percent of them are girls, while 17 percent are children with disabilities, the report found.
Another nearly 120 million children living in crisis-prone areas attend school but in such difficult conditions that they do not achieve minimum proficiency in mathematics or reading.
Other children may be attaining the minimum proficiency, but cannot reach their potential due to lacking services like school meal programmes or psychosocial support for the many struggling with trauma, Sherif explained.
The analysis found that 84 percent of the fully out-of-school children were living in areas with protracted crises, including in Afghanistan, the Democratic Republic of Congo, Ethiopia and Yemen.
The Russian-Ukraine conflict since February 24 has meanwhile pushed millions more children out of school.
The most recent estimate indicates the conflict has impacted around 5.7 million school-aged children, ECW pointed out.
Sherif voiced particular concern over the situation in Afghanistan, where girls have been all but excluded from secondary education since the Taliban regained control of the country last August.
"It's very important that the world now speaks up," she said.
"We cannot allow Afghan girls in secondary education not to come back to school."