Six out of 10 children and teenagers in the world are failing to reach basic levels of proficiency in learning, warns a hard-hitting report from the United Nations.
The UN describes the findings as "staggering" and representing a "learning crisis".
Much of the focus of international aid in education has been on the lack of access to schools, particularly in poorer countries in sub-Saharan Africa or in conflict zones.
But this new research from the Unesco Institute for Statistics warns of the lack of quality within schools - saying more than 600 million school-age children do not have basic skills in maths and reading.
In sub-Saharan Africa, the research suggests 88% of children and adolescents will enter adulthood without a basic proficiency in reading.
And in central and southern Asia, 81% are not reaching an adequate level in literacy.
The report warns any ambitions for social and economic progress will be stifled without a literate and numerate population.
In North America and Europe, only 14% of young people leave education at such a low level. But, the UN research suggests, only 10% of the world's school-age children live in these more affluent, developed regions.
"Many of these children are not hidden or isolated from their governments and communities - they are sitting in classrooms," said Silvia Montoya, director of the Unesco Institute for Statistics.
She said the report was a "wake-up call for far greater investment in the quality of education".
This problem of "schooling without learning" was also highlighted by the World Bank in a report this week.
It warned that millions of young people in low- and middle-income countries were receiving an inadequate education that would leave them trapped in low-paid and insecure jobs.
The president of the World Bank, Jim Yong Kim, introducing the report, said the failures in education for so many represented "a moral and economic crisis".
Researchers warned of pupils in Kenya, Tanzania, Uganda and Nicaragua who after years in school were unable to do simple sums or read simple sentences.
A basic level of proficiency in primary school was reached by 99% of pupils in Japan, but by only 7% of pupils in Mali, they said.
There were also wide gulfs within countries. At the end of primary school in Cameroon, only 5% of girls from the poorest families were at a level to continue with their education, compared with 76% of girls from wealthy families, the report said.
The World Bank study examined the factors underlying such poor achievement:
The World Bank's chief economist, Paul Romer, said there had to be a more honest admission that for many children being in school did not mean worthwhile lessons.
He said progress would depend on recognising that "the facts about education reveal a painful truth".
Lack of testing
The report warned of a lack of scrutiny over standards and the absence of even basic information about pupil achievement.
While the debate in Western countries has been about excessive testing, the World Bank said that in poorer countries, there was "too little measurement of learning, not too much".
But the researchers also pointed to countries that had made progress, such as South Korea and Vietnam.
And at the United Nations last week there were international pledges for greater investment in education.
"I have decided to set education as a top priority of French development and foreign policy," said French President Emmanuel Macron.
Former UK Prime Minister and UN education envoy Gordon Brown said he wanted the Global Partnership for Education, which channels aid to education projects, to have funds worth $2bn (£1.5bn) by 2020.
The European Union announced that 8% of its humanitarian budget would be spent on education.
For children missing school because of the conflict in Syria, the Education Above All Foundation and Unicef, along with other charities, committed an extra $60m (£45m).
"Funding our education goal will do far more than place a child at a desk. It will unleash opportunity and hope," said Mr Brown.