In some countries, fewer than half of youngsters are learning the basics, Unesco's annual monitoring report concludes.
It warns that billions of dollars are being spent on poor quality education that is failing to ensure children learn.
The United Nations agency said more good teachers were needed, and called on governments around the world to provide enough decent, trained teachers to ensure all children get a good education.
The latest Education for All Global Monitoring Report calculates that the cost to governments of around 250 million children not learning the basics is an estimated 129 billion US dollars (around £80bn).
This equates to 10% of global spending on primary education.
Figures suggest fewer than half of children are learning basic English and maths in 21 out of the 85 countries with complete data available.
Of these, 17 countries are in sub-Saharan Africa, the others are India, Mauritania, Morocco and Pakistan.
A new analysis conducted for the study found around 175 million young people - equivalent to one in four youngsters - cannot read all or part of a sentence.
Unesco director-general Irina Bokova said: "We need 5.2 million teachers to be recruited by 2015, and we need to work harder to support them in providing children with their right to a free and quality education."