Schools have started sending entire year groups home for Christmas as the Government warns against breaking up early.
The National Education Union (NEU) said schools are starting to send year groups home because of a rise in teachers who are isolating.
Some schools have closed down for a few days as a "circuit breaker", while others have sent hundreds of children home until the start of the new term in January.
On Wednesday evening, Boris Johnson said children should not be taken out of school before the end of term and nativity plays should go ahead.
He told a Downing Street press conference: "We don't want children to be taken out of school before the end of term, not that there is very long to go now. We don't want nativity plays to be cancelled."
Schools struggle to find supply staff
Kevin Courtney, the NEU's joint general secretary, told The Telegraph: "We are hearing there are schools where the number of staff who are having to isolate because they have tested positive is increasing and the schools cannot find supply staff.
"There are some schools that have had to close for some year groups because they haven't got the staff, and others worrying about when this will happen to them."
Officials at the Department for Education (DfE) insisted on Thursday that face-to-face education should continue and that school attendance remains mandatory.
But several schools around the country have started to shut their doors and instruct children to learn remotely for a period of time.
Arlecdon Primary School, in Cumbria, announced on Thursday that it would be closing for a five-day "circuit breaker" following advice from local public health officials.
Kingswood Secondary Academy in Corby, Northamptonshire, sent two year groups home until the start of next term because of "staff shortages due to illnesses".
In a letter to parents, Michelle Newman, the school principal, said all pupils in years seven and eight should remain at home and would receive remote education until the Christmas holidays.
Todholm Primary School, in Paisley, Renfrewshire, is also closed due to "the impact of self-isolation on school staff numbers". It is due to reopen on Monday after five days of remote learning.
Pinner High School, in north London, has sent several children home to self-isolate after they were identified as close contacts of a suspected omicron case.
Council 'strongly recommends' cancelling carol services
Croydon Council, in south London, has written to headteachers to "strongly recommend" that all Christmas carol services and performances should be cancelled unless they take place outdoors, and that year group bubbles should be reintroduced.
Rachel Flowers, Croydon's director of public health, told teachers on Dec 1 that the measures were necessary because "we are seeing cases rise markedly in school-age children". Headteachers have been told to "revisit" outbreak plans and ensure contingency plans are up to date so they are "prepared for any future changes".
Ofsted inspections will not go ahead in the final week of term to ensure schools and colleges in England can plan for omicron contingency measures, the DfE said.
Meanwhile, Oxford University told students there had been a "significant rise" in the omicron variant and has asked them to get a PCR test at a walk-in mobile testing unit in the city and take daily lateral flow tests while they await their result.
Any students who plan to leave the city should take two lateral flow tests three days apart and report their results to the university.
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