Parents have been urged not to mix outside school gates as millions of pupils return to the classroom.
But a leading children’s doctor warned parents and pupils not to mingle when arriving at and leaving school.
Professor Russell Viner, president of the Royal College of Paediatrics and Child Health, said: “The key thing is that children themselves, and parents, don’t think, ‘The schools are open, we can relax, we can mix outside of school.'”
He told Times Radio on Monday that schools can only open safely if everything else “stays locked down” for at least three weeks, and conceded that reopening schools will lead to a rise in coronavirus cases.
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He sais: “We know that reopening schools will increase transmission, but we should be able to keep the R [reproduction rate] below 1 – that’s the key thing to stop the runaway increase of infections.”
Prof Viner also spoke about the impact of children being out of school for so long.
“We closed down our children’s lives,” he said.
"The key issues around meeting friends, development socially, learning to trust, learning to be human, learning all of those things, that’s been lost as well as the loss of actual straight-up learning, and many of those things can’t be done online.
“The harms to mental health are very clear. We know that there’s a lot more anxiety and depression and eating disorders around among children and young people.”
Under the first phase of the government’s road map out of lockdown, visits to care homes are also resuming from Monday and people can meet one other person outdoors for a coffee or picnic.
Boris Johnson said that although the changes are “only a small relaxation of the rules”, they will bring “joy and relief” to families after months of “tough restrictions”.
Professor Calum Semple, a member of the Scientific Advisory Group for Emergencies (Sage), said schools are “absolutely” safe for children and that the main risks come from the increased contacts among adults that will inevitably follow.
“The main driver is not the pupil-teacher relationship,” he told BBC Breakfast.
“When we talk about schools, it is the fact that the school brings adults together, whether that’s teaching staff, the domestic staff, the catering staff, and it’s an opportunity for mixing.”
Education secretary Gavin Williamson has said he is looking at proposals including a five-term academic year, a shorter summer holiday and longer school days to help pupils catch up on learning lost during the pandemic.
Secondary-school pupils, who are likely to have their return staggered over the week to allow for mass testing, are being asked to take three voluntary COVID-19 tests on site and one at home over the first fortnight. They will then be sent home-testing kits to use twice weekly.
The Department for Education is also advising secondary-school students to wear face coverings wherever social distancing cannot be maintained, including in the classroom.
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