Save the Children on Tuesday called for urgent action to help kids in the world's poorest countries to return to schools closed as part of coronavirus restrictions.
The British charity said it would cost just $370 (£275) per pupil to get youngsters back, warning that every day without lessons risked widening inequalities.
London-based Save the Children has repeatedly called for action to help children continue their education despite the pandemic, which its chief executive Inger Ashing called "the biggest education emergency we've ever seen".
The charity published its latest research looking at the world's 59 poorest countries, including Uganda, Syria and Yemen, calculating the average cost of reopening schools with anti-virus safety measures as well as helping children catch up lost lessons.
The total cost for all 136 million children affected would be $50 billion, it estimated, urging international donors to help governments raise the sum.
This would cover financial support for families to send children to school, catch-up classes, hygienic facilities and safety training for teachers, as well as national campaigns to encourage children to return, it said.
"The priority should be the poorest and most marginalised like girls, refugees and displaced children and children with disabilities," the charity said.
Such children are otherwise likely never to return to education, it said, warning of a "lost generation".
In Uganda alone, more than 13 million children have been out of schools since March, according to Save the Children.
"Children who are currently out of school are our future doctors, scientists, truck drivers, and plumbers," Ashing said.
"If 2020 was the year of finding the vaccine, 2021 needs to be the year of investing in children's future."
The charity stressed it is not advising countries when to reopen schools, however, and is providing distance-learning materials.
