Pupils in years 7 and above should wear face masks in communal areas, the Department for Education has told schools in England.
The advice is also in place for visitors to schools in England.
It comes after the prime minister brought in new rules on face coverings following the detection of the Omicron variant in the UK.
The new COVID variant is thought to be more transmissible and health officials have warned about the possibility of the new strain being able to evade vaccines, although it is currently too early to tell if this is the case.
From Tuesday wearing masks will be mandatory in shops and public transport – bringing England in line with other parts of the UK. Pubs and restaurants will be exempt.
Other measures, which will come into force as soon as possible, include anyone identified as a contact of someone with the Omicron variant having to self-isolate for 10 days – regardless of vaccination status.
Anyone who enters the UK from abroad will be required to take a PCR test and self-isolate until they get a negative result.
At a Downing Street news conference, Boris Johnson described the measures as “temporary and precautionary” – and said they will be reviewed in three weeks’ time.
Only British and Irish citizens will be allowed into the UK from some countries in southern Africa that have been newly added to the rest list.
They will have to show a negative test taken within three days of departure and spend 10 days in a government-approved quarantine hotel.
Secondary schools are also facing additional COVID precautions after the Christmas break.
All students will be asked to take a rapid lateral flow test on site when they return in January.
In an email to schools on Friday, the Department for Education told leaders to order enough testing kits by Tuesday to make this possible.
Testing all students “will help reduce transmission after a period of social mixing over the school holidays”, the correspondence said.Internet Explorer Channel Network