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.
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.
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."
Children wearing face masks in classroom
Masks are now compulsory again in classrooms for secondary pupils in England
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.