Members of the armed forces are to give remote support to secondary schools and colleges in England setting up mass Covid testing as the new term begins.
Military personnel will hold webinars and give phone support to school staff.
But head teachers say they need support on the ground and more time to make the plan workable. They are calling for a delay to the start of term.
The government wants pupils to go back in the first two weeks of January, but is keeping the situation under review.
Under the current plans, the majority of secondary school pupils in England are due to start term on 4 January studying remotely, to give head teachers time to implement a round of coronavirus testing for students and staff.
Primary school students will return as normal during the same week and will not be tested for Covid-19.
Those in exam years and vulnerable pupils will return in person first, with the rest expected to go back on 11 January.
But this entails 5.5 million secondary pupils being tested in schools in the space of a week.
Several media reports suggest ministers are considering delaying the start of term, with potentially only exam year pupils starting remotely in the first week of January.
In response to the reports, a spokesman for the Department for Education said: “We want all pupils to return in January as school is the best place for their development and mental health.
“But as the prime minister has said, it is right that we follow the path of the pandemic and keep our approach under constant review.”
Geoff Barton, general secretary of head teachers’ union, the Association of School and College Leaders, said it seemed a “sensible and prudent approach” to delay the return to school, given the rising incidents of Covid and the new strain of the virus.
He added: “What is vital is that schools are given clear guidance as soon as possible so they can make the necessary arrangements.”
The Ministry of Defence said 1,500 personnel will form local response teams, providing support and phone advice to institutions on setting up and running Covid testing facilities.
Support would be offered “predominantly through webinars and individual meetings”, it said, but teams would also be on standby to provide in-person support at short notice.
Mr Barton said schools had been hoping for “boots on the ground” in schools and colleges to help run testing centres.
“Instead, it appears that there will be 1,500 personnel available for more than 3,400 secondary schools in England, and that the support will be mainly in the form of webinars and meetings.
“This is not remotely sufficient to support schools in the huge task they are being asked to undertake by the government.”
And head teacher of Royal Wootton Bassett Academy in Wiltshire, Anita Ellis, reiterated that schools needed “bodies on the ground to help deliver the testing”.
“We don’t know where we are going to get the volunteers from, we don’t know how we are going to get the vetting done in time,” she said. “Really what we need is on the ground support.”
There is also pressure from other teaching unions for schools to move to online learning for all pupils except those deemed vulnerable or children of key workers.
A meeting was held between ministers, Downing Street officials and the Department for Education on Monday to discuss the plan further, but the department would not comment on its outcome. Some teaching union leaders are understood to be meeting DfE officials this afternoon.
The military has previously helped organise mass testing in Liverpool and more recently in Kent to clear the backlog of lorries caused by France shutting its UK border.
Students will swab themselves in the vast majority of cases, under the supervision of school staff or a volunteer who has been trained for the role, and teachers are not expected to take a role in the testing process.
The idea in schools is that following one round of testing all pupils, school staff would be offered a test every week. Then anyone who is a close contact of someone testing positive would be offered tests every day for seven days.
This would cut the amount of school pupils miss due to self isolation, and help keep the virus out of schools.
Schools in all the UK nations are remaining open for vulnerable children. England, Wales and Scotland have also committed to maintain face-to-face teaching for children of key workers.
Early research by the London School of Hygiene and Tropical Medicine suggests schools and universities might need to close on top of existing tier four restrictions to bring the new fast-spreading coronavirus variant under control.
However, Conservative MP Robert Halfon, who chairs the Commons Education Committee, said keeping children in schools must be the government’s “priority”.
He said: “What needs to happen is volunteers – perhaps the armed forces, perhaps mobile units outside schools or in school playgrounds – making sure pupils and teaching staff are tested and also rolling out vaccinations as a priority for all those in schools.”