Mr Johnson said he understood people’s concerns about children returning for the new term but said education is “a priority”.
Speaking on The Andrew Marr Show on BBC One, the Prime Minister said: “Schools are safe. It is very, very important to stress that.
“The risk to kids, to young people is really very, very small indeed.
“The risk to staff is very small.
“I would advise all parents thinking about want to do, look at where your area is, overwhelmingly you’ll be in a part of the country where primary schools tomorrow will be open.”
He added: “I understand people’s frustrations, I understand people’s anxieties but there is no doubt in my mind that schools are safe and that education is a priority.”
Gavin Williamson confirmed on Friday that all London primary schools will remain shut to most pupils next week – rather than just those in certain boroughs as set out earlier in the week – but teaching unions say all schools should close for the next two weeks.
On Saturday evening, the Department for Education said remote learning was “a last resort” and classrooms should reopen “wherever possible” with appropriate safety measures to help mitigate the risk of transmission.
Hundreds of new vaccination sites are due to be up and running this week as the NHS ramps up its immunisation programme with the newly approved Oxford University and AstraZeneca jab.
Some 530,000 doses of the vaccine will be available for rollout across the UK from Monday and more than a million patients have already had their first dose of the Pfizer/BioNTech vaccine which was the first to be approved by the Medicines and Healthcare products Regulatory Agency.
But Ofsted chief inspector Amanda Spielman, writing in the Sunday Telegraph, said: “It is clear that children’s lives cannot just be put on hold while we wait for vaccination programmes to take effect, and for waves of infection to subside.
“We cannot furlough young people’s learning or their wider development. The longer the pandemic continues, the more true this is.”
Children’s Commissioner Anne Longfield told the newspaper that schools should be the last to close and first to open, when safe to do so, adding: “I hope, for children and parents’ sake, that is measured in days not weeks and I would be particularly keen for primaries to stay open if at all possible.”
Labour leader Sir Keir Starmer said the vaccine roll-out was “our great hope”, adding: “I want the Government to throw everything it can at this, harnessing the extraordinary talents of our NHS so we can be vaccinating at least two million Brits a week by the end of the month.”
But, writing in the Sunday Mirror, he criticised “a chaotic last minute U-turn on schools”, adding: “Confusion reigns among parents, teachers and pupils over who will be back in school tomorrow and who won’t.”
General secretary of the National Education Union (NEU) Dr Mary Bousted earlier said schools should stay closed for two weeks to “break the chain” of transmission and prevent the NHS becoming overwhelmed.”
The union, which represents the majority of teachers, has advised its members it is not safe to return to classrooms on Monday.
NAHT general secretary Paul Whiteman said the union had started preliminary steps in legal proceedings against the Department for Education (DfE), asking it to share its scientific data about safety and transmission rates.
He told BBC Breakfast: “What we’re talking about is understanding the risks. Having a short break so that we can agree the right mitigations in schools to make them Covid-secure, make sure that staff and teams are vaccinated and that we can get a properly supported testing regime in schools to make them as safe as possible.
“And then have an orderly return to school that’s sustainable, rather than the chaos that we have experienced throughout the pandemic, with the DfE making last-minute decisions because they didn’t take proactive action.”
Unions have also called for the reopening of schools in Wales next week to be delayed with Laura Doel, director of school leaders’ union NAHT Cymru, saying “the latest data shows that in large parts of Wales, control of infection has been lost”.
From January 4, all London primary schools will be required to provide remote learning for two weeks to all children except vulnerable children and those of key workers, who will be allowed to attend.
Mr Williamson said the January 1 decision to expand closures to the nine remaining London boroughs and the City of London was a “last resort”.
Under the Government’s initial plan, secondary schools and colleges were set to be closed to most pupils for the first two weeks of January, while primary schools within 50 local authorities in the south of England, including 23 London boroughs, were also told to keep their doors shut until January 18.
Green Party-led Brighton and Hove City Council has advised primary schools in the Tier 4 area not to return in person, except for vulnerable children and those of key workers, until January 18 despite the Government’s plan for most schools to open in person.
Linda Bauld, a professor in public health at the University of Edinburgh, told the PA news agency that transmission among primary school pupils was “still very limited” while secondary school pupils, particularly older teenagers, can pass on the virus in the same way as adults.
But health professionals have warned of growing pressure on services with Professor Andrew Goddard, president of the Royal College of Physicians, telling the BBC current case figures are “fairly mild” compared to what is expected in a week’s time.
Among those receiving hospital treatment after catching Covid-19 is shadow culture secretary Jo Stevens, Labour MP for Cardiff Central.
Confirmed cases were higher than 50,000 for the fifth day in a row when UK figures were released on Saturday with a record-high of 57,725 lab-confirmed cases and another 445 deaths within 28 days of a positive test.