Secondary school pupils in England returned from the Christmas break this week to new advice - to wear face coverings in lessons and to take lateral flow tests at school before heading into classrooms.
These are recommendations and are not mandatory. There are no statistics yet that show how many pupils have been complying, but some parents and pupils have expressed concern that a number are choosing not to.
Some charities say they are worried about the effects on vulnerable students of this advice being ignored, but some parents say masks are causing problems for children's learning.
The government says schools "can decide how best to encourage" pupils to take tests and wear masks.
The Department for Education (DfE) stressed that pupils should not be "denied education" on the basis of whether or not they wear a mask.
It has previously said it is determined to keep schools open, despite more staff absences being expected.
'We don't need to wear masks'
Molly, a Year 8 pupil in West Yorkshire, whose full name and school we are not identifying, told the BBC that some children in her school were refusing to wear face coverings.
"A teacher will say 'can you please put your masks on?' And they'll just say 'no, we don't need to wear masks, we don't want to wear them'," she said.
"There were a few people the other day saying that when we passed them in the corridor."
She said it made pupils feel "more unsafe", and that vulnerable classmates were at particular risk.
Steve, in Lincolnshire, said his 16-year-old daughter texted him when she returned to school to say that "quite a few" pupils were not wearing face coverings.
He said she was was "slightly worried" - as he and his wife are key workers, and his father had been seriously ill in hospital with Covid in 2020.
"I'm slightly surprised that they haven't got the ability to enforce the situation," he said.
Another father, who asked not to be named, said children at his son's school refusing to wear masks reflected a "lack of leadership" from head teachers.
'Hard to concentrate'
However, other parents were concerned about the effects of masks in classrooms on learning.
A mother in Kent said pupils in her son's school had been wearing them since September, because of high Covid case numbers.
"He says it's hard to concentrate with long periods of time in a mask," she said.
She said she was pleased they were introduced, but wanted children to be given more fresh air breaks to remove them.
"This really will have an impact [on learning] for years to come," she said.
A DfE spokeswoman said the temporary advice to wear face coverings in lessons aimed "to help reduce transmission and disruption to learning and maximise attendance". She pointed out that pupils of secondary school age and above are advised to test twice a week at home.
"Schools know their pupils best and can decide how best to encourage them to wear face coverings and take Covid tests," she said.
"No-one should ever be denied education on the grounds that they are, or are not, wearing a face covering."
The government's own study in the autumn did not provide proof of a statistically significant impact of wearing masks in schools.
The evidence review says other studies have provided mixed results, but taken together suggest masks may help.
Damien McNulty, from the NASUWT teachers' union, told the BBC that schools should engage with parents and pupils to encourage uptake.
He said there were "huge numbers" of pupils refusing to wear masks and take lateral flow tests in six secondary schools in the north-west of England.
At one school in Lancashire, only 67 children out of 1,300 were prepared to do either, he said - while another in Manchester said "there was no point" in offering lateral flow tests when pupils returned.
Mr McNulty said this was likely to be the case in other parts of the country.
There was an element of teenagers "railing against" authority, he added, made worse by some wanting to "push boundaries" with supply teachers in charge.
Some charities have urged school pupils to be mindful of vulnerable classmates.
Susan Walsh, chief executive of Immunodeficiency UK, said those affected by immunodeficiency may not want to disclose their condition.
"Those refusing to do lateral flow tests and wear face masks will put them at severe risk," she said.
"I am sure that most children wouldn't intentionally want to put anyone in harm's way, so I hope that they will rethink their stance and show solidarity in protecting others with underlying medical conditions."
Children with underlying health conditions have been offered vaccinations since the end of the summer.
The government classes vulnerable children as those with specific conditions including severe neuro-disabilities, Down's Syndrome, underlying conditions resulting in immunosuppression, profound and multiple learning disabilities, sickle cell disease, type 1 diabetes and congenital heart disease.
Rachael Olley, chief executive of the Childhood Cancer Parents Alliance, said she imagined refusal to wear masks and take tests would be "of concern to both patients and their parents or carers".
She pointed to research from the UK Paediatric Oncology Coronavirus Monitoring Project suggesting that Covid "does not present a greater risk" to children receiving cancer treatment, but added: "That does not stop parents from worrying."
Julie McCulloch, of the Association of School and College Leaders, pointed out that some pupils would be exempt from wearing masks and "the reasons for not wearing a face covering may not be visible to others".
She added: "Clearly, this presents schools and colleges with a very difficult area to navigate in determining what are legitimate reasons and what are not legitimate reasons.
"The majority of parents and pupils support the policies of their school or college in this respect, and in cases where there are issues, this will generally be dealt with by way of appropriate conversations."