As the government appeared to lose control of the rebellion, parents across England received emails announcing their primary school would not open as planned because of staff safety concerns.
Within hours Mr Johnson had announced that all primary and secondary schools and colleges in England would close from tomorrow until February half-term, except for vulnerable children and those whose parents are key workers.
In a televised statement from Downing Street, the prime minister also acknowledged that the closures mean it will not be “possible or fair” for all exams to go ahead as planned this summer.
Education secretary Gavin Williamson is to consult with regulator Ofqual before a formal announcement, but the PM’s words appeared to signal the replacement of many A-level and GCSE exams with some form of student assessment this summer.
Mr Johnson said he understood the distress the late change would cause millions of parents and pupils.
And he acknowledged: “Parents whose children were in school today may reasonably ask why we did not take this decision sooner.
“And the answer is simply that we’ve been doing everything in our power to keep the schools open, because we know how important each day in education is to children’s life chances.”
The general secretary of the Association of School and College Leaders (ASCL), Geoff Barton, said headteachers were “relieved the government has finally bowed to the inevitable and agreed to move schools and colleges to remote education in response to alarming Covid infection rates”.
But he added: “This new lockdown will be disruptive to learners who have already been hugely disrupted. Schools and colleges are very worried about how exams can be made fair in these circumstances.”
Patrick Roach, the leader of the NASUWT teaching union, said it was right for the PM to respond to unions’ calls for remote learning for pupils nationwide.
“We have seen too much disruption to children’s education, but without tougher measures that disruption was only set to continue,” said Dr Roach.
He called on ministers to “learn lessons” from previous lockdowns and engage with teachers now to develop robust and credible plans for reopening safely and avoid a repeat of last year’s qualifications fiasco.
It was “extremely disappointing” that education staff were not being given priority for vaccines, he said, adding: “Keeping teachers free from Covid is the best way to ensure that children’s education does not continue to be disrupted.”
Primary and secondary schools in a number of “hot spot” areas, including London, were closed indefinitely last week.
But on Sunday morning the prime minister told parents in the rest of England they should prepare to send their children to class, claiming it was “safe”.
Within hours the UK’s largest teaching union, the National Education Union, said 6,000 teachers had informed their schools they would not be going to work because of safety concerns.
Before the prime minister’s announcement Pete Bowdery, a teacher from Surrey, told The Independent he believed all schools should remain physically shut.
“I believe a two- to four-week lockdown where schools are shut in a controlled and planned manner is far better for children’s progress and mental health than a situation like last term, where entire year groups were sent home with less than three hours’ notice,” he said.
While schools remained open last term, more than half a million state school pupils were off school during the final weeks of term for coronavirus-related reasons, Department for Education (DfE) estimates show.
“The short-term pain of a closure would surely mitigate future pain caused by the hokey-cokey style schooling seen by so many over the last few months,” Mr Bowdery said.
Labour leader Sir Keir Starmer said he understood the “anger and frustration” felt by parents and teachers, but said the school closures were “inevitable” as part of a national plan to rein in the virus.
“What we now need, I think, is proper support for working parents, who will be really concerned about what to do in the coming days and weeks, proper support for children at home, because that package needs to be in place, and we need to already start working on how we can reopen again,” Starmer told the BBC.
The closure of schools marks another humiliation for Mr Williamson.
Just over a week ago friends of the education secretary told a Sunday newspaper he faced a battle against the teaching unions and others to reopen schools this month.
Within days, however, Mr Williamson had been forced to announce major changes to the start of the school year, including that most secondary school pupils would not return until 18 January and that all schools in ‘hot spot’ areas of infection would remain closed indefinitely.
But there was widespread anger when it emerged that those hots spots included most of London and involved more than half a million primary schoolchildren.
Before the latest announcement education unions had called on ministers to pause the reopening of schools for all except vulnerable children or the children of key workers.
“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,” a group of half a dozen unions said.