The number of children out of school with a confirmed case of COVID-19 topped 100,000 in England last week, according to government figures.
The Department for Education (DfE) found the number of pupils out of school for coronavirus-related reasons increased by two thirds in a fortnight.
More than 204,000 – 2.5% of all pupils – were not in class for reasons connected to COVID-19 on Thursday last week.
This is up from 122,300 children, or 1.5% of all pupils on 16 September – a 67% rise from two weeks ago.
The figures come as heads reported “a high level of disruption”, with a school leaders’ union warning that self-isolation rules are “actively contributing” to the spread of the virus in schools.
The 204,300 figure includes 102,000 pupils with a confirmed case of COVID-19, up from 59,300 on 16 September, and 84,100 with a suspected case, up from 44,600.
About 11,400 were absent due to isolation for other reasons, down from 15,900 on 16 September.
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A further 4,800 pupils were off due to attendance restrictions being in place to manage an outbreak, up from 2,000, and 2,000 did not attend as a result of school closures due to COVID-19, up from 500.
Last week, Education Secretary Nadhim Zahawi said the government would not “stand back and let attendance fall” as education is “simply too important”.
But the latest analysis of pupil attendance shows that 89.5% of students were in class on 30 September, compared with 91.9% on 16 September.
Geoff Barton, general secretary of the Association of School and College Leaders (ASCL), said the “grim statistics show a big increase in the number of pupils out of school as a result of the continuing havoc caused by coronavirus”.
“We are hearing from schools where there are 10% or more of pupils absent and where staff are also off work because of the virus.
“Teaching and learning is very difficult in these circumstances and it is clear that the educational disruption of the past 18 months is far from being over.”
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Mr Barton has called on the new education secretary to urgently set out what action he intends to take to address the situation.
He added: “One thing he might do is to look at why it is taking so long to deliver the carbon dioxide monitors to schools that the government promised at the start of term.”
Schools in England no longer have to keep pupils in year group “bubbles” to reduce mixing, and children do not have to isolate if they come into contact with a positive case of COVID-19.
Instead, they are advised to get a PCR test and only isolate if they test positive.
In Wales, the government has announced that from next Monday, secondary school and college pupils will be advised to take daily lateral flow tests for seven days if someone in their household tests positive for coronavirus.