Schools could be told to break up early for Christmas over fears the arrival of the Omicron variant may fuel an explosion in infections.
Reports say thousands of children have already been told to switch to remote learning as increasing numbers test positive.
More and more members of staff are also said to be catching the virus - which may force education bosses to act soon.
But the Department for Education (DfE) has said that sending large groups of kids home should be a "last resort".
Three cases of the Omicron variant have so far been discovered in the UK - in Nottingham, Essex and central London.
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This prompted the DfE to issue guidance on mask wearing on Sunday, strongly advising secondary school pupils to do so in communal areas.
An email update said: "Face coverings should be worn in communal areas in all settings by staff, visitors and pupils or students in Year 7 and above, unless they are exempt.
"Pupils or students (in Year 7 or above) should continue to wear face coverings on public and dedicated school transport, unless they are exempt."
Secondary schools in England have also been ordered to provide on-site testing for after the Christmas holidays.
Heads were told on Friday to make sure they have enough supplies so all pupils can be given a lateral flow test when they return in January.
In an email, the DfE said it would "help reduce transmission after a period of social mixing" over the festive period.
It said: "We would like all secondary schools to prepare to test their pupils once on-site on return in January.
"We understand that this is a significant additional ask but testing continues to play a vital role in keeping Covid-19 out of schools."
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Education secretary Nadhim Zahawi said: "The news of a new variant – the so-called Omicron variant – will have understandably caused concern for people across our country, including our teachers, wider education and childcare staff, parents, pupils and students.
"We are already taking targeted and proportionate action as a precaution while we find out more information about the new variant.
"As we do so, we will continue to prioritise children’s and young people’s education and wellbeing, making sure education and childcare settings are as safe as possible and children continue to benefit from classroom teaching.
"We are working with education and childcare settings to enhance safety measures where needed, including introducing isolation for 10 days for close contacts of suspected Omicron cases."
