Schools and colleges in England are to be closed to most pupils until at least half term, Boris Johnson has announced.
The prime minister said the new lockdown had to be "tough enough" to stop the variant virus from spreading - and teaching will go online.
A-Levels and GCSEs will be cancelled, a government source confirmed to BBC News - although vocational exams will go ahead.
The National Education Union accused the government of causing "chaos".
In a television address, Mr Johnson announced the biggest changes to schools since the early days of the first lockdown in March.
'Not fair for exams'
"Because we now have to do everything we possibly can to stop the spread of the disease, primary schools, secondary schools and colleges across England must move to remote provision from tomorrow," said the prime minister.
This means a return to online learning for pupils of all ages - apart from vulnerable children and the children of key workers who can continue to go into school.
"We recognise that this will mean it's not possible or fair for all exams to go ahead this summer, as normal," said Mr Johnson.
It is understood that vocational exams will continue, but GCSEs and A-levels will be cancelled - and that the exam watchdog Ofqual will make "alternative arrangements" for delivering results.
An attempt to produce replacement exam grades last summer turned into one of the biggest U-turns of the pandemic.
Teachers' unions accused the government of failing to react more swiftly to "mounting evidence" about Covid transmission in schools and to make preparations for remote teaching and alternatives to written exams.
'Head in the sand'
But Mary Bousted, co-leader of the National Education Union, said Education Secretary Gavin Williamson had "become an expert in putting his head in the sand".
Geoff Barton of the ASCL head teachers' union criticised ministers for having issued legal threats to keep schools open at the end of last term - and then "made a series of chaotic announcements about the start of this term".
The new term, which began on Monday for primary pupils, has only lasted a day before it has been suspended.
The prime minister said he hoped that schools would be "reopening schools after the February half term".
There have been assurances that there will be a more thorough approach to home learning than in the first lockdown last year.
The Department for Education has provided hundreds of thousands of computer devices - with the aim of supporting those without the equipment needed to work online from home.
There have also been suggestions Ofsted inspectors will play a more active role in checking on what support schools are providing to pupils in their online learning.
Universities in England had already planned a staggered return for this term - but there will now be even fewer students on campus this month.
The latest lockdown guidance says university students who are taking hands-on courses such as medicine or veterinary science should return for face-to-face lessons as planned.
These students will be expected to take two Covid tests or self-isolate for 10 days when they return.
But students on all other courses are being told not to come back to university if possible and to start their term online "until at least mid-February".