The government is under pressure to give schools in England more flexibility over allowing face masks.
Head teachers, who want an urgent review of guidance against masks, said if there is going to be a u-turn it should be “sooner rather than later”.
Prime Minister Boris Johnson said, depending on the medical advice, “if we need to change the advice then of course we will”.
In Scotland secondary pupils will have to wear masks in school corridors.
In England, despite official guidance against face coverings, some schools are already preparing to use them.
The Oasis academy trust, with more than 50 schools in England, is providing visors for its teachers – and secondary pupils will have to wear masks in corridors.
On Tuesday, the ASCL head teachers’ union said parents and schools needed more clarity and “reassurance” over the evidence for not allowing masks.
The head teachers say it would be “prudent” to reconsider the opposition to allowing masks, “in the light of the decision in Scotland over face coverings in schools, and the recent guidance by the World Health Organisation“, which recommended face coverings for the over-12s.
The heads’ union asked what schools should say if teachers or pupils say they want to wear a face covering – and for a “clear direction one way or another”.
The union’s leader Geoff Barton also urged “if there is going to be any U-turn by the government that it does this sooner rather than later, because the start of the new term is imminent”.
On a visit to Devon, the prime minister emphasised the safety of the return to school in September, saying “all our scientific advice is that schools are safe, it’s absolutely crucial people understand that”.
On whether the guidance on face coverings, Mr Johnson 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.”
But Business Secretary Alok Sharma said earlier on Tuesday that “Public Health England does not recommend face coverings in schools”.
However, Mr Sharma said he did not expect sanctions for schools which decided to use masks.
“I don’t think we want to go down the road of penalising people. This is about what makes sense,” he told BBC Radio 4’s Today programme.
Despite the guidance against using masks, it seems some schools are already making their own decisions.
Steve Chalke, chief executive of the Oasis academy 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 help to protect teachers as well as pupils, he said, including those who might have underlying health conditions or who are 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 will be provided, might increase the confidence of parents “nervous” about sending their children back to school.
A 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.
The government’s safety guidance, issued in July, says Public Health England does not recommend using face coverings in school.
As pupils would be in their own separate “bubbles” there is no need for masks, says the guidance, which warned that “misuse” of face coverings could “inadvertently increase the risk of transmission”.
A Department for Education spokeswoman said: “We have consistently followed Public Health England advice, which does not recommend the use of face coverings in schools because there are a range of protective measures in place, including children staying in consistent groups.
“We have set out the system of controls schools should use, including cleaning and hygiene measures, to substantially reduce the risk of transmission of the virus when they open to all children in the coming weeks.”