The Department for Education wrote to headteachers to urge them to prepare to test pupils in school at the start of next term
Schools in England have been ordered to provide on-site testing for secondary pupils after the Christmas holidays.
Heads were told to make sure they have enough supplies so all pupils can be given a lateral flow test when they return to the classroom in January.
In an email to schools on Friday, the Department for Education (DfE) said it would “help reduce transmission after a period of social mixing” over the festive period.
But union leaders accused the Government of treating heads like “an ad hoc branch of the NHS “.
It comes as Britain braces to see the impact of a worrying new Covid variant first detected in South Africa, which has now been found in the UK.
Two cases of the Omicron variant have been identified in Essex and Nottingham, prompting Boris Johnson to front a hastily-arranged press conference on Saturday announce new measures to halt its spread.
Face coverings will be mandatory in shops and on public transport again from Tuesday, while all international arrivals will soon be required to take a more costly PCR test and isolate at home until they get their result.
All contacts of an Omicron case will have to isolate at home for 10 days, regardless of their vaccination status.
Around 130,000 pupils (1.6%) in state schools were out of class due to Covid on November 11, down from 248,000 (3.2%) on October 21, according to the DfE.
But some schools have been forced to shut temporarily or send children home due to absence rates among staff and pupils.
The Government guidance says schools should only send large numbers of children home in “extreme cases” and only as a “last resort”.
On Friday, the DfE emailed school leaders to advise them to prepare for mass testing after Christmas.
The email 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.”
Secondary schools have also been asked to order home testing kits so pupils can continue to test.
The email said: “Testing all pupils in school boosts testing participation and will help reduce transmission after a period of social mixing over the school holidays.
“Tests, PPE and funding to support your workforce will be provided as before. After this test on return, pupils should continue to test in line with Government guidelines.”
Julie McCulloch, director of policy at the Association of School and College Leaders (ASCL), said: “We recognise the importance of Covid testing to reduce the risk of transmission of the virus, but it is not reasonable for the Government to once again impose this considerable public health task on schools with minimal support.
“The role of schools should be limited to providing a space for test centres and communicating with students. But the Government expects them also to handle the logistics of staffing and setting up testing stations.
“It seems to have forgotten that school leaders are educators rather than an ad hoc branch of the NHS.
“Their focus is on providing the teaching and learning required by their students, which is particularly important in the context of the disruption caused by the pandemic.
“The last thing they need is another huge responsibility which does not even fall within the remit of education.”
At the start of the autumn term, all secondary school and college pupils in England were invited to take two lateral flow tests at school on their return.
Pupils were then asked to continue to test twice weekly at home.
A DfE spokesperson said: “As part of our commitment to protecting face-to-face education, we are asking all secondary school pupils to complete one on-site test as they return to school after the Christmas break, reflecting that mixing between households is likely to increase over Christmas.
“Protective measures in place continue to strike a balance between managing transmission risk with testing, hygiene, ventilation and vaccines, and we encourage any eligible young people who haven’t yet had the vaccine to consider using the Christmas break to do so.”