In mid-July, schools were closed in 160 countries across the world, affecting over 1 billion students, the United Nations has said.
As many as 40 million children missed out on education in their critical pre-school years.
UN chief Antonio Guterres issued a stark warning on Tuesday that the disruption caused by the coronavirus pandemic to children's education could lead to a "generational catastrophe".
"We are at a defining moment for the world's children and young people,'' Guterres said in a video message. "The decisions that governments and partners take now will have a lasting impact on hundreds of millions of young people, and on the development prospects of countries for decades to come.''
Despite the best efforts of teachers to keep education going through distance learning and parents trying to home school their children, many cannot be reached. The pandemic has accentuated inequalities with disadvantaged communities, refugees and displaced people at higher risk of being left behind.
Even before the pandemic, the UN estimates that 250 million school-age children were out of school and only a quarter of secondary school children in developing countries were leaving school with basic skills.
The UN chief called for schools to reopen and welcome students in a safe way as soon as possible. He urged governments to increase their spending on education and address what he calls the "digital divide".
"Education initiatives must seek to reach those at greatest risk of being left behind — people in emergencies and crises, minority groups of all kinds, displaced people and those with disabilities," he added.
Germany was the first European country to reopen schools this week as the summer holidays are coming to an end in several states. Strict hygiene rules have been put in place with some schools making masks mandatory inside.
In the United States, pupils are also starting a new year and President Donald Trump has urged schools to reopen despite an uptick in COVID-19 cases in several parts of the country.
On Monday, the country's top infectious disease expert, Doctor Anthony Fauci, warned that "there may be some areas where the level of virus is so high that it would not be prudent to bring the children back to school.”