There are 61 million children of primary school-age (6-11 years) disadvantaged, 60 million of lower secondary school-age (12-14 years), and upper secondary school-age (15-17 years) set at 142 million.
The findings are part of the 2030 Agenda for Sustainable Development report released jointly by the UNESCO Institute for Statistics and the Global Education Monitoring (GEM) Report.
The report is based on data from 128 countries over the period 2010- 2015.
Irina Bokova, the director-general of UNESCO, urged the global community to undertake a common effort to overcome the educational "failure".
"There are today 264 million children and youth not going to school - this is a failure that we must tackle together, because education is a shared responsibility and progress can only be sustainable through common efforts.
"According to the report representing 90 percent of the global population of upper secondary school age, less than one in four young people had completed upper secondary school in 40 countries and less than one in two in 60 countries ... there were only 14 countries with a completion rate of at least 90 percent," she said
"Governments are the primary duty bearers for the right to education, yet this right is not justiciable in almost half of countries, and the primary course of action for those with a complaint is lost," she added.
She said stronger mechanisms should be established to tackle the failure and also "to enshrine and enforce the right to education and hold all governments to account for their commitments, including donors".
Source: News agencies