The United Nation says there is a pressing need for 69 million teachers to be recruited around the world by 2030 to reach 230 million non-schooling children around the world, warns Unesco.
The United Nations agency’s estimate is for the number of teachers required to meet the promise of primary and secondary places for all children.
The biggest gaps in staffing are in sub-Saharan Africa and southern Asia.
The Unesco report says there needs to be a “seismic shift” in recruitment to overcome “massive shortages”.
At present, the report from the Unesco Institute for Statistic says there are about 263 million children without a primary or secondary school to attend.
This includes about 25 million children who will never set foot inside a school of any kind.
World leaders last year agreed a set of global targets for access to education, as part of the Sustainable Development Goals.
But the report says that keeping this promise for primary and secondary school for all by 2030 will require a huge increase in teachers.
The most acute pressure is in sub-Saharan Africa, where countries would need to train another 17 million teachers to meet the demand.
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The Unesco study warns that there are already shortages of teachers in these countries, as they struggle with rising populations.
“Without urgent and sustained action, the situation will deteriorate in the face of rising demand for education,” says the report.
The report identifies countries where staffing numbers are “getting worse, rather than better”, including Burundi, the Central African Republic, Kenya, Malawi and Mozambique.
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Image caption Conflicts, such as that in South Sudan, have destroyed schools and children’s chances of an education
But there are a number of countries in the region on track to have enough staff to meet the targets, including Ethiopia, Rwanda and Swaziland.
The study highlights the importance of the quality as well as the quantity of teachers.
In countries such as Niger, Ghana, Sierra Leone and Liberia, fewer than 60% of teachers in primary school have been trained.
Pauline Rose, professor of international education at Cambridge University, says the lack of teachers also affects class sizes, particularly when the population is rising.
“So in countries like Malawi, it is common to find over 100 children in classes in the early grades of primary school. This has been a persistent problem for many years.”
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Image caption Teachers’ strike in Costa Rica: Pay remains an issue for teachers in many countries
The lack of pupils completing secondary school in some countries in sub-Saharan Africa further compounds the shortage of teachers.
Prof Rose says that in “some countries around half of secondary school graduates would need to go into teaching to fill the teacher gap, which is clearly not viable”.
There is also a problem with low pay which makes teaching less attractive, particularly to the most able graduates.
Prof Rose says that in some countries in sub-Saharan Africa teachers are “paid below the poverty line, and so not surprisingly they take second jobs to compensate. This has an impact on the quality of their teaching”.
And she says there might be enough teachers trained on a national level, but there could still be local shortages in regions where schools cannot get teachers to apply.
The Unesco report highlights the importance of retaining staff and says this will require teaching to have a competitive salary and contracts which will give them job security.
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Image caption Classroom in Liberia: Teaching needs to offer a competitive salary to recruit staff, says Unesco
Vikas Pota, chief executive of an education charity, the Varkey Foundation, said: “We already know that better pay will attract the best graduates into the profession and give them an incentive to stay.
“A 10% increase in teachers’ pay tends to result in a 5% to 10% increase in pupil performance. Given the stretched finances of developing world governments, the international community has a responsibility to help fund this.”
The Varkey Foundation has been experimenting in Ghana with interactive distance learning to try to offer training on a wider scale than would be possible with in-person classes. It is training up to 5,000 teachers over two years.
The UN report says the success of global targets for education will depend on tackling the teacher shortage.
“Such efforts could falter if they fail to prioritise those on the front line: the world’s teachers, who are tasked with the actual delivery of a good quality education for all.”
Last month, there was another warning from Unesco about the delays in creating enough primary school places by the 2030 target.
A report warned that at current rates of progress it would be 2042 before all primary-age children would be able to attend school.
The most limited access to schools was found in countries which were the poorest or most troubled by conflict.
Niger, South Sudan, Burkina Faso, Afghanistan, Mali and Chad were among the nations whose children were likely to spend the least time in education