Nearly 260 million children missed out on school in 2018 and the coronavirus pandemic has only exacerbated the problem, according to UNESCO.
The agency's 2020 Global Education Monitoring Report says poorer children, girls, the disabled and immigrants are among those at a disadvantage, and that the situation got worse with COVID-19 when more than 90 percent of the world's schoolchildren found their learning affected by closures.
While those from better-off families had internet and wifi connections and were able to use laptops and mobile phones, millions of youngsters were left out.
"Health crises can leave many behinds, in particular the poorest girls, many of whom may never return to school," wrote Audrey Azoulay, UNESCO's director-general.
The report found 258 million children and young people were entirely excluded from education, with poverty as the main obstacle to access. In low- and middle-income countries, adolescents from the richest 20 percent of all households were three times as likely to complete lower secondary school as were as those from the poorest homes.
It said the pandemic was an opportunity for change.
"COVID-19 has given us a real opportunity to think afresh about our education systems," said Manos Antoninis, Director of the Global Education Monitoring Report. "But moving to a world that values and welcomes diversity won’t happen overnight. There is an obvious tension between teaching all children under the same roof and creating an environment where students learn best. But, COVID-19 has shown us that there is scope to do things differently if we put our minds to it."