Pupils across the UK are heading back to class amid fears of staff shortages and remote learning.
More Covid-related staff and pupil absences are expected this term as schools reopen following the Christmas break.
Teaching unions say it is likely some classes and year groups will be sent home to learn remotely at times.
But England’s Education Secretary Nadhim Zahawi insists face-to-face teaching will remain the norm.
The UK’s pupils are returning to class this week, with in-school testing taking place in England.
Elsewhere, pupils are testing themselves for Covid at home.
The government ruled on Sunday that pupils should wear masks in classrooms for the first three weeks of term, bringing England in line with other parts of the UK.
It also pledged 7,000 air filters would be available to the 20,000-plus schools in England.
Liberal Democrat Education spokeswoman Munira Wilson described the investment as “pocket change” compared to what was required to keep schools open.
On Monday, Mr Zahawi said the government had learned a “painful lesson” about the impact of previous school closures. This was why he and the prime minister were determined to keep schools open, he said.
Before term ended, the education secretary appealed to former teachers to come out of retirement and sign up with teacher supply agencies to help cover staff absence. There has been a similar call to retired staff in Northern Ireland.
However, supply agencies have warned it is unlikely those responding the to the call will be in place for the start of term.
Association of School and College Leaders general secretary Geoff Barton said: “The biggest problem schools face is the likelihood of high levels of staff absence caused by the prevalence of the Omicron variant.
“While schools and colleges will do their very best to minimise the impact on pupils, as they always do, there is a possibility that this will mean that some classes and year groups have to be sent home for short periods of time to learn remotely.”
Paul Whiteman, general secretary of the National Association of Head Teachers, said teachers and school staff will be testing and reporting their results at the start of this week.
“Only then will school leaders know who they have available and be able to properly plan.”
Stephen Brierley, principal of St Margaret’s Church of England Academy in Liverpool, said if schools, such as his, were unable to get cover staff – which they had been finding difficult before Christmas – they would have no choice but to close to a number of year groups.
Oak National Academy, the national online school which was set up in the first school lockdown of 2020, says it is ready for increased demand.
It has emailed advice to schools on how cover staff can download slides, worksheets and quizzes and even “team-teach”, using the videos as needed.
Its principal Matt Hood said: “Our support staff stand ready to advise on how to set cover lessons and prepare for combined classes should staff shortages rise.”
On Sunday, six trades unions representing teaching staff issued a joint statement calling for more government support in keeping schools Covid-safe.
They want easily accessible funds to cover the cost of the temporary staff who are likely to be needed, support with the testing regime and exemption from Ofsted inspections so they can focus on keeping schools safe.
They warned national exams could be at risk if “urgent and immediate action” was not taken to minimise Covid-related disruption.
The Department for Education responded by ordering face masks be worn in England’s classrooms, as they were in the other three UK nations.
It also says scheduled exams will go ahead and highlighted that 99.9% of schools remained open in the autumn term.
But the department also acknowledged that the Omicron variant was “expected to cause increased staff absence levels in the spring term”.
Last month, more than 30 local authorities told the BBC that some classes had moved online at schools because staff were sick or isolating, and they could not get supply teachers to replace them.
Across the UK, staff and secondary pupils are being urged to test themselves regularly, and schools have been asked to ensure there is good hygiene and ventilation.
In England, secondary pupils are being invited into schools for a Covid test, and staff are being asked to test before they go back. Twice-weekly testing at home will continue after this.
There is a further push for staff and more young people to get vaccinated.
The latest official survey of vaccination rates suggest half of households with children in Great Britain have had their Covid jabs.
In Scotland, schools are being asked to minimise contacts between pupils by using “groupings” and social distancing and face coverings are being used.
In Wales, all schools have been told to use the first two days back from the Christmas holidays as “planning days” to “assess staffing capacity and put the necessary measures in place”, with pupils due to return by 10 January at the latest.
Face coverings are advised in communal areas and classrooms.
In Northern Ireland, staff and older pupils are being asked to take a lateral flow test within 24 hours of returning, and schools have been warned they may need to switch to remote learning for short periods.
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