The World Health Organization (WHO) has issued guidance saying children over the age of 12 should wear masks, in line with recommended practice for adults in their country or area.
It admits little is known about how children transmit the virus but cites evidence that teenagers can infect others in the same way as adults.
Children aged five and under should not normally wear masks, the WHO said.
More than 800,000 people have now died with coronavirus worldwide.
At least 23 million cases of infection have been registered, according to Johns Hopkins University, with most of them recorded in the US, Brazil and India.
However the true number of people who have had the virus is believed to be far higher, due to insufficient testing and asymptomatic cases.
The numbers have been rising again in countries as diverse as South Korea, EU states and Lebanon.
WHO chief Tedros Adhanom Ghebreyesus has said he hopes the pandemic will be over in two years but a top scientific adviser in the UK warned Covid-19 might never be eradicated, with people needing regular vaccinations.
The advice published on the WHO website covers three age groups:
For teachers, the WHO says: "In areas where there is widespread transmission, all adults under the age of 60 and who are in general good health should wear fabric masks when they cannot guarantee at least a one-metre distance from others.
"This is particularly important for adults working with children who may have close contact with children and one another."
Adults aged 60 or over, or those with underlying health conditions, should wear medical masks, it says.
The WHO guidance does not specify whether a child over the age of 12 should wear a mask in school, but it may yet become a feature of the classroom as the new academic year begins.
France recently made it mandatory for all children over 11, and a number of schools in the UK are taking it upon themselves to require students to wear them even though this is not official government guidance.
James Gillespie's High School in Edinburgh is one such school, making the decision to require students to "wear face coverings indoors whilst moving around between classes" after taking feedback from pupils, staff and parents. Nicola Sturgeon, the first minister of Scotland has warned secondary school students may be required to wear face coverings in the "near future".