Some 264 million school-age children and young people worldwide were not in education in 2015, the United Nations culture and education agency UNESCO said on Tuesday.
The agency, in a progress report on the UN's development goals for education, said that after a decline in the early 2000s, out-of-school rates have started to stagnate.
Worldwide, there was a completion rate of 83 per cent for primary education, falling to 45 per cent for upper secondary schooling, the agency said, quoting household survey data from 128 countries for the 2010-2015 period.
There were 40 countries where fewer than one in four young people had completed secondary education, but only 14 where at least 90 per cent had done so.
UNESCO Director General Irina Bokova called for more government accountability.
The report noted that while 82 per cent of national constitutions mention a right to education, only 55 per cent of countries make that right enforceable in the courts.
"Governments are the primary duty bearers for the right to education, yet this right is not justiciable [capable of being the basis for a court case] in almost half of countries, and the primary course of action for those with a complaint is lost," Bokova wrote.
While calling for accountability at all levels, the report said accountability measures for schools needed to be flexible and carefully designed.
"Schools may adjust to performance-based accountability systems in negative ways, gaming the system and avoiding sanctions to the exclusion of longer-term reforms," the agency warned.