Education unions have called for the reintroduction of extra safety measures in schools after official estimates showed around 270,000 secondary pupils had COVID-19 last week.
The demand for action came as an expert warned about the level of coronavirus circulating among older children.
The latest figures from the Office for National Statistics (ONS) show that around one in 15 children in school years 7 to 11 in England are estimated to have had COVID-19 in the week to 2 October.
This was the highest positivity rate for any age group and up from one in 20 during the previous seven-day period.
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Kevin McConway, emeritus professor of applied statistics at the Open University, said that with around four million children in the 11 to 16 age group in England, one in 15 equates to around 270,000 testing positive.
He said based on separate ONS estimates, this means about 13,500 could go on to have long COVID symptoms, and almost 7,000 may have symptoms that limit their day-to-day activity
Prof McConway added: “That’s rather a lot of children who will have their schooling and other activities disrupted.
“I think the main necessary action here is to get on with vaccinating those aged 12-15 as quickly as possible – though, given that it takes time for the vaccine to become effective, that’s still not going to bring down the infection rate in this group all that fast.
“I’m certainly not suggesting general school closures or anything like that, but I don’t think it’s an option to do nothing.”
His comments came as five education unions wrote to Education Secretary Nadhim Zahawi, calling on him to look at bringing back stricter safety measures for schools.
The GMB union, Unite, Unison, National Education Union (NEU) and NASUWT teaching union said they are also writing to local authorities and directors of public health.
Kevin Courtney, joint general secretary of the NEU, said: “We are concerned that the government is standing by while COVID cases surge across schools.
“It is evident that more needs to be done, and sooner rather than later, to prevent further massive disruption to children’s education, caused either by children contracting COVID-19 or COVID-related staff absence.”
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Jim Kennedy, Unite national officer for education, called on Mr Zahawi to “reset the safety agenda for schools”.
He argued that with winter approaching, “the whole range of measures to keep school children safe needs to be deployed – the rising level of infections in schools demand it”.
The NASUWT said schools need more support with on-site testing, “rather than relying on home testing, which is less effective”, and called on the government to consider reinstating the requirement for pupils who are contacts of a positive case to self-isolate.
Avril Chambers, GMB national officer, said the latest figures show “it is clear further mitigation measures are needed immediately” in order to avoid further disruption to schooling.