Video report by ITV News Political Correspondent Paul Brand
As part of the government's new lockdown in England, schools will close their doors until February 15.
Meanwhile, Scotland announced that schools will remain shut until at least February and Northern Ireland and Wales have delayed the beginning of term.
Here's what we know so far:
How will coronavirus restrictions affect pupils in the UK?
The prime minister has announced that all pupils across England – except children of key workers and vulnerable pupils – will move to remote education from Tuesday until the February half-term. Special schools and nurseries will remain open.
In Scotland, First Minister Nicola Sturgeon said all schools will revert to remote learning until at least February.
In Wales, schools will move to remote learning until January 18 and in Northern Ireland, the new term will be delayed by a week. Northern Ireland First Minister Arlene Forster has not ruled out keeping schools closed following a rise in the number of coronavirus infections.
Why are schools set to close?
The decision to close schools came after education unions warned that bringing all pupils back to class could fuel the pandemic and put teachers at “serious risk” of falling ill amid the new variant of Covid-19.
The government’s "chaotic" handling of the opening of schools has caused confusion for parents and teachers, according to unions representing school staff and headteachers.
All of London’s schools and those in some surrounding areas worst hit by Covid-19 had not yet returned to class, but primary school pupils elsewhere in England began heading back to school on Monday.
Secondary schools and colleges had already been told they would have a staggered return, with those taking exams expecting to return on January 11 and other year groups on January 18.
Mr Johnson said he recognised the “inconvenience and distress” that the late change would cause millions of parents and pupils up and down the country.
Speaking on Monday, he said: “Parents whose children were in school today may reasonably ask why we did not take this decision sooner.
“The answer is simply that we have been doing everything in our power to keep schools open, because we know how important each day in education is to children’s life chances.”
Mr Johnson added: “And I want to stress that the problem is not that schools are unsafe for children – children are still very unlikely to be severely affected by even the new variant of Covid.
“The problem is that schools may nonetheless act as vectors for transmission, causing the virus to spread between households.”
Will exams take place as planned?
School exams look set to be cancelled, and Boris Johnson said pushing ahead with all exams this summer “as normal” would not be fair or possible.
In his televised address to the nation on Monday evening, Mr Johnson said Education Secretary Gavin Williamson will work with exams regulator Ofqual to put in place “alternative arrangements”.
Announcing the change, the PM said: “We recognise that this will mean it is not possible or fair for all exams to go ahead this summer as normal.”
Last month, Education Secretary Mr Williamson gave an “absolutely” cast-iron guarantee that exams in England would not be cancelled this academic year.
The pledge came as the government unveiled a package of measures in December for students sitting their A-level and GCSE exams in the summer to compensate for disruption to their schooling during the pandemic.
Pupils in England were told they would be given advance notice of some topics ahead of their tests and they would be allowed to take in exam aids, like formula sheets, to ensure this cohort of students is not disadvantaged.
Now the Department for Education and Ofqual will discuss the future of GCSE and A-level exams in England.
However, BTEC exams which are due to take place in the next few days will go ahead, the government said.
What support will there be for disadvantaged children?
The PM vowed that pupils entitled to receive free school meals will continue to receive them while learning from home. And he pledged more laptops will be provided to low-income families to support learning.
He said: “We will provide extra support to ensure that pupils entitled to free school meals will continue to receive them while schools are closed, and we will distribute more devices to support remote education.”
What restrictions are there on higher education?
University lessons will remain online until mid-February for all except future critical worker courses.
Students living at university should not move between their student home and permanent home during the term time.
It is the latest blow to university students after thousands last year were "trapped" in self-isolation after coronavirus outbreaks in at least 30 universities.
Students undertaking training and studying for some courses, however, can return to face-to-face learning after they are tested for the virus or self-isolate for ten days.
Medicine & dentistry
Subjects allied to medicine/health
Education (initial teacher training)
Courses which require Professional, Statutory and Regulatory Body (PSRB) assessments and or mandatory activity which is scheduled for January and which cannot be rescheduled.
Those eligible for face to face teaching can meet in groups outside of their household as part of their education or training but follow social distancing guidelines.
How have education unions reacted to the latest measures?
Geoff Barton, general secretary of the Association of School and College Leaders (ASCL), said: “This decision clearly raises a question about GCSEs, A-levels and other exams which are due to take place in the summer.
“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.”
He added: “We are keen to work with the Government and exam regulator Ofqual to make this process as fair as it can possibly be.”
Patrick Roach, general secretary of the NASUWT teachers’ union, said: “Given the scale of the difficulties faced, the government is right to look at alternative plans for exams this summer.
“It is essential that the government works with the profession on these plans and does not repeat the mistakes of last summer. The government must also confirm that it is cancelling statutory tests for primary school pupils.”
Dr Mary Bousted, joint general secretary of the National Education Union (NEU), said: “No one wanted schools and colleges to be shut again but the evidence clearly pointed to the necessity for this to happen weeks ago.
“Why Boris Johnson allowed such confusion and chaos to build up around school openings before making this belated, blindingly obvious decision is beyond belief.
“Government must take responsibility for this closure because it has allowed Covid-19 to become, again, out of control.”