Face masks should be worn in the classroom where it is impossible for secondary students to keep two metres apart, the Government has said.
Ministers are recommending the use of face masks in "all indoor environments" in secondary schools, colleges and universities – including during lessons – where social distancing cannot be maintained.
Boris Johnson said the measure would be in place for secondary pupils for the rest of this term to offer "even greater reassurance" that face-to-face teaching is safe.
That goes much further than earlier Government recommendations on face masks in secondary schools. During the autumn term, official guidance said face masks should be worn in corridors and communal areas in parts of the country under Tier 2 or Tier 3 restrictions. Elsewhere, it was left to the discretion of headteachers.
The more stringent mask recommendations came as Mr Johnson confirmed that all primary school children will return to the classroom on March 8, with all secondary pupils going back from the same date.
The return of secondary pupils will start on the same date but may be phased over a week to allow for a mass testing programme to take place on site, the Government said. Secondary school students will be asked to take four lateral flow tests during the first two weeks of term, three of which will take place at school and one will be taken at home.
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Students will be allowed to continue coming to school as long as their tests are negative, but will be asked to go home and isolate if they have a positive result.
They will then also be asked to take two lateral flow tests each week at home and report their results to teachers at school. If they get a positive result, they will need to immediately isolate and follow it up with a confirmatory PCR test.
Headteachers criticised the Government's plan to get pupils back to the classroom, arguing that the return should be phased over a longer period of time.
Geoff Barton, the general secretary of the Association of School and College Leaders, the main union for secondary heads, said mass testing is a "huge logistical challenge" and may mean pupils' return should be "staggered over a minimum of two weeks".
He said that while he wants children to be back in lessons as soon as possible, a "full return" on March 8 "may prove counterproductive and lead to more disruption" if it leads to Covid rates rising again and another round of potential school closures.
Meanwhile, the National Education Union, the country's biggest teacher union, urged the Government to publish the science and modelling that underpins the decision to reopen schools.
The National Deaf Children's Society criticised the new recommendations on face masks in the classroom, saying it would have a "devastating" effect on deaf children's studies, mental health and ability to take part in lessons.
Ian Noon, the charity's head of policy, said: "With England’s 35,000 deaf pupils close to a return to education, the goalposts on face masks have moved yet again. We cannot have a situation where thousands of deaf children and young people are unable to understand their teacher, leaving many with little point in even attending class."
University students were told they could return to university on March 8 if they are undertaking degrees with a practical element or if they need specialist facilities. But all other students must remain at home at least until the end of term, with ministers promising to review guidance by the end of the Easter holiday.
Children's after school clubs and activities will be allowed to resume from March 8 where they are necessary for childcare purposes. Outdoor sports for youngsters will also be able to start on the same date if it is part of their "education provision" or for childcare reasons.
Papers published on Monday by the Government's scientific advisers warned reopening schools is "likely" to cause the 'R' rate – the number of people a single infected person will go on to infect – to rise above one.
The Children's Task and Finish Group, a sub-committee of the Scientific Advisory Group for Emergencies (Sage), said the "consensus view" is that the reopening of schools is "likely to increase effective 'R' by a factor of 1.1 to 1.5", equivalent to a 10 per cent to 50 per cent rise.
"In further modelling of increasing cohorts of pupils returning to school, the relative impact on 'R' increases as additional cohorts of children return," they said. "Transmission to children and young people can occur in household, community and educational settings."
When schools reopen there is a knock-on effect as it may enable parents of younger children to return to the workplace.
The Sage papers explained that the return of children to the classroom "cannot be viewed in isolation" and must be considered in the context of the broader trajectory of the epidemic.
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