The UK government said teachers were not at greater risk of catching coronavirus than the rest of the population, as it faced defiance from schools in England that chose to remain closed.
Some schools opted not to reopen on Monday following the Christmas break after unions raised safety concerns.
The disagreement came a day after Prime Minister Boris Johnson sent mixed messages on school closures. He said there was "no doubt in my mind that schools are safe", but he also said the faster-spreading coronavirus variant meant tougher measures were required.
The government did not impose a blanket ban on schools reopening, delaying the return of England’s secondary schools until later in January while keeping primary schools open.
But former health secretary Jeremy Hunt said schools must close "right away" as the pressure faced by hospitals was "off-the-scale worse" than previous winter crises.
Education unions said schools returning made no sense when the infection rate remained at record levels. On the day the Oxford/AstraZeneca vaccine was distributed to UK sites there were also calls for teachers to be among the first immunised.
On Sunday, 54,990 positive tests and 454 deaths were recorded in the UK.
According to the latest data, one in four people tested for Covid-19 in London were infected with the virus. A positive test rate of 26.8 per cent was recorded on December 29, the latest available data, up from 15.7 per cent on December 22.
“Bringing all pupils back into classrooms while the rate of infection is so high is exposing education sector workers to serious risk of ill-health and could fuel the pandemic," unions said.
They called for a pause in the reopening of schools for anyone other than vulnerable children and children of key workers, and a move to remote learning for all while Covid-secure working arrangements are reviewed.
"All school staff continuing to work in schools should be given priority access to Covid-19 vaccinations.”
Proud to sign this joint statement from @the_tuc and TUC unions representing education staff.
— Frances O'Grady (@FrancesOGrady) January 4, 2021
Everyone must be safe at work.
The PM must talk to trade unions about what steps are needed to make sure all schools are Covid-Secure.https://t.co/HtYidD1Zm8
Health Secretary Matt Hancock said that teachers did not face a higher risk than other key workers.
"The proportion of teachers who catch coronavirus is no higher than the rest of the population, so there is clear public health advice behind the position that we take and that is what people should follow because, of course, education is very important as well, especially for people's long-term health," he told BBC Radio 4's Today programme.
Mr Hancock’s comments and the scientific basis behind the government’s strategy were called into question.
The latest scientific advice on infection rates in schools was presented on December 17 but the paper acknowledges the data was collected before the more infectious Covid-19 strain was identified.
Dr Michael Tildesley, a modelling expert who advises the government’s Sage advisory group, said new measures would need to be brought in but he warned: “Even if you closed schools, that might not be enough to bring the R number below 1.”
Paul Whiteman, general secretary of school leaders' union NAHT, said mixed messages from politicians and scientists meant it was "very hard to tell" how many schools would be open for the start of the term.
"Some schools in Tier 4 areas will be open for vulnerable pupils and key-worker families and will be providing remote learning for others, yet in other areas also in Tier 4, all pupils will be admitted," he said.
"That's a confusing picture for school leaders and families alike."
Labour leader Sir Keir Starmer said it was "inevitable" more schools would have to close to curtail the spread of coronavirus.
Mr Johnson is also facing pressure within his own ranks.
Jeremy Hunt tweeted: “To those arguing winter is always like this in the NHS: you are wrong. I faced four serious winter crises as health sec and the situation now is off-the-scale worse than any of those.”
