Schools in England may reopen region by region after half term, the government's deputy chief medical officer Jenny Harries has said.
Speaking to the Commons education committee, Dr Harries suggested there would be different rates of infection across the country when lockdown ends.
This would mean a "differential application" of restrictive measures would be required, she said.
Schools were closed at the start of January to stem the spread of Covid-19.
Although schools remain open to vulnerable children and those of keyworkers, all others are due to learn remotely from home until after the February half term holiday.
But the Health Secretary, Matt Hancock, has suggested they may not return fully then.
A Department for Education spokesperson said the department was continuing to keep plans for the return to school under review and that it would inform schools, parents and pupils of the plans ahead of February half term.
Committee chairman Robert Halfon said he suspected schools would be closed for quite "a few weeks yet", but there has been no formal confirmation of this.
Medical and science advisers were warning the government before Christmas that the NHS would not be able to manage the number of Covid-19 cases if schools remained open.
'Uneven spread'
The new, more transmissible variant of the virus had been increasing exponentially in London and the south-east before Christmas.
But in some parts of the north and north-east saw rates of increase were reducing.
Dr Harries said: "It is highly likely that when we come out of this national lockdown we will not have consistent patterns of infection in our communities across the country.
"And therefore, as we had prior to the national lockdown, it may well be possible that we need to have some differential application."
But Dr Harries said schools would be at the top of the priority to ensure that the balance of education and wellbeing were "right at the forefront" of consideration.
Geoff Barton, general secretary of the Association of School and College Leaders, said: "Although the government intends that schools will fully reopen after the February half-term holiday, it is clearly in the balance when this happens and whether there will be any sort of regional approach.
"We expect that it will depend on coronavirus infection rates and the pressure on the NHS, and that the government will make a call on this issue nearer the time.
"What is important is that when schools fully reopen, everything possible is done to keep them open and to keep disruption to a minimum.
"This is why we are calling for education staff to be prioritised for vaccinations as soon as possible, and for schools to be given more support in the use of rapid turnaround mass testing."
'Clarity needed'
Paul Whiteman, general secretary of the National Association of Head Teachers, said if the government was planning to stagger opening of schools by region, it needed to "provide clarity sooner rather than later".
"This will give vital time to prepare for a smoother reopening of schools and business," he said.
On calls for vaccination of teachers, Dr Harries suggested the safe re-opening of schools did not depend on this.
But members of the committee suggested education would be less disrupted by teachers needing to go home and isolate when infected.
The vaccination programme had been worked out in order of vulnerability to the disease, she stressed.
And Dr Harries added that although pupils could and did transmit the virus, she did not have evidence of them being "a significant driver" of "large-scale community infections".
Pressure grows on PM to reopen schools amid warning children are pandemic's 'forgotten victims'
The prime minister and education secretary are coming under increasing pressure to reopen schools.
Education Secretary Gavin Williamson is expected to confirm this week that there will be no return to the classroom after the February half-term break and Health Secretary Matt Hancock has already refused to guarantee that schools would reopen before Easter.
Rob Halfon, the chairman of the Commons Education Committee, said he had written to Speaker Sir Lindsay Hoyle asking him to summon a minister to Parliament on Monday for an urgent question.
Mr Halfon said on Twitter that he was seeking "clarity" from the Department for Education and "an education route map out of coronavirus to get children learning again at school".
Speaking to Sky News, he said he wants ministers to set out a "route map" for the reopening of schools.
Mr Halfon added that teachers and support staff should be "made a priority" for vaccines once the elderly and clinically vulnerable have had a jab.
Former cabinet minister Esther McVey said the government should take into account the damage the prolonged closures were doing to the future prospects of a generation of children.
"We genuinely seem to have forgotten about the children," she told The Daily Telegraph.
"Millions of them are missing out on an education, not developing socially with their friends and aren't allowed to enrich their lives by playing sports and music any more.
"They are the pandemic's forgotten victims and we've got to start thinking about their prospects and futures as well."
And Tory MP Tom Tugendhat said: "Closed schools increases inequality, exposes the most vulnerable, and creates gaps that cannot be filled. We must open schools as soon as possible."
Work and Pensions Secretary Therese Coffey told Sky News the government recognised that the current situation was "challenging for the children, the teachers, for parents".
She added: "I can assure you that the prime minister is as keen as possible to try and get back to face-to-face learning as quickly as possible.
"We know that schools are safe for children, but it's about the wider impact and conscious that children could end up acting as a vector."
It comes after the health secretary told Sky News on Sunday that teachers have a "good shout" to be "very high" on the next priority list for a coronavirus vaccine.
So far, more than five million people have had their first dose - with the UK government and devolved administrations aiming to hit 15 million by mid-February.
Only vulnerable pupils and children of key workers are currently able to attend school, and in a blow to parents, Mr Hancock said he wasn't sure if schools in England will reopen by Easter.
Asked if he could promise they will, he told Sky's Sophy Ridge On Sunday: "We have got to look at the data, we have got to look at the impact of the vaccination programme.
"The education secretary has said that we will ensure schools get two weeks' notice of return.
"I don't know whether it will be then or before then. We have got to watch the data."
It comes as The Sunday Times reported the government is preparing to rule out children returning to the classroom after the February half-term holiday, with the prospect of home schooling continuing for several months.
The paper quoted a government source as saying: "We are in this for the long haul."
A Department for Education spokesperson said: "We continue to keep plans for the return to school under review and will inform schools, parents and pupils of the plans ahead of February half term."
They added: "We will continue to work to reopen schools as soon as possible."