Face coverings should be worn by secondary pupils and staff in local lockdown areas of England, and will be at the discretion of secondary schools across the country, after the Government made another U-turn.
The Department for Education (DfE) advised that face coverings should be worn when moving around corridors and communal areas .
They will not need to be worn in classrooms, because other protective measures will already be in place and they might affect learning, it added.
Face coverings will not be recommended in schools more widely, the department said, but added that schools will have the discretion to require them to be worn by staff and children in Year 7 and above in communal areas.
The change follows World Health Organisation (WHO) advice that children aged over 12 should wear masks, the Government said.
Just this week Education Secretary Gavin Williamson insisted measures being adopted by schools to limit the spread of coronavirus meant masks were not required.
But announcing the change in advice on Tuesday, he said: "Our priority is to get children back to school safely. At each stage we have listened to the latest medical and scientific advice.
"We have therefore decided to follow the World Health Organisation's new advice. In local lockdown areas children in year 7 and above should wear face coverings in communal spaces.
"Outside of local lockdown areas face coverings won't be required in schools, though schools will have the flexibility to introduce measures if they believe it is right in their specific circumstances.
"I hope these steps will provide parents, pupils and teachers with further reassurance."
Full guidance has not yet been published, but is expected "shortly" the department said.
In a warning that advice could change again, the DfE said stricter guidance could be issued for schools nationally if the rate of transmission increases across the country.
Earlier Prime Minister Boris Johnson had hinted that guidance may change, as pressure grew on ministers from teaching unions and after Holyrood confirmed secondary school pupils in Scotland will be required to wear face coverings in between lessons, from Monday.
Mr Johnson told reporters the Government will "look at the changing medical evidence as we go on", adding: "If we need to change the advice then of course we will."
Post-primary pupils in Northern Ireland will be asked to wear face coverings in corridors and other communal areas, Stormont's education minister said on Tuesday.
The official guidance comes into effect on August 31 when schools there return on a full-time basis.
Welsh health minister Vaughan Gething said a decision on schoolchildren wearing face coverings will be made on Wednesday, but current guidance says masks are not being recommended.