Secondary pupils in England will have to wear masks in school corridors in local lockdown areas of England, after the government reversed its guidance.
Head teachers in any secondary school will also have the "flexibility" to introduce masks in their schools.
Education Secretary Gavin Williamson says it follows updated advice from the World Health Organization.
"At each stage we have listened to the latest medical and scientific advice," said Mr Williamson.
The Department for Education says it is keeping its recommendation against using face coverings - but that schools can make their own decision to ask pupils and staff to wear them.
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This will be in "communal areas" of schools such as corridors, rather than in classrooms, when schools "believe that is right in their particular circumstances".
The government had been under pressure over face masks in England's schools - with secondary schools in Scotland to use them in corridors or shared areas from next week.
Northern Ireland is also now advising masks for secondary school corridors. In Wales, a decision on updating the guidance is due on Wednesday.Prime Minister Boris Johnson had earlier said: "We'll look at the changing medical evidence as we go on, if we need to change the advice then of course we will."
The ASCL head teachers' union had warned of confusion about the rules over face coverings - and said there was a lack of clarity over how schools should respond if teachers or pupils wanted to wear masks.
They had said it would be "prudent" to reconsider guidance against using masks - in the light of a recent recommendation from the World Health Organization for over-12s to wear face masks.
The union's leader, Geoff Barton, had urged that if there was a U-turn, that it should be "sooner rather than later".
Despite the official guidance against face coverings, some schools had already been preparing to use them.
Image copyright Getty Images Image caption Teachers and pupils wearing masks in a classroom in GermanyThe Oasis academy trust, with more than 50 schools in England, is to provide visors for its teachers - and secondary pupils will have to wear masks in corridors.
Steve Chalke, chief executive of the trust, said there was a responsibility to make schools "as safe as we possibly can" - and that meant using masks and visors.
Like wearing a school uniform, he said wearing masks would become "part of what we do" and would be part of a wider safety plan, including hand washing and keeping pupils apart in separate "bubbles".
But in secondary school corridors and shared areas, where pupils could not be kept apart, Mr Chalke said masks would be required.
"We're using our common sense for our schools and our buildings," he said.
It would be another safety measure for teachers, he said, including those who might have underlying health conditions or in a more vulnerable age group.
Mr Chalke said schools leaders had to make a "moral choice" on how best to make schools safe on the scientific advice available.
He also thought using masks, which would be provided, might increase the confidence of parents "nervous" about sending their children back to school.
Image copyright Liam McBurney Image caption A teacher in Northern Ireland wearing a visor as pupils return to schoolA secondary school in Kent, Sir Roger Manwood's School in Sandwich, is expecting pupils and staff to wear masks - with pupils each being given three washable masks.