Five schools have been forced to shut down and hundreds of pupils are in self-isolation after coronavirus outbreaks.
The Samuel Ward Academy in Haverhill, Suffolk, closed on Public Health England advice after five teachers returned positive tests.
Meanwhile, Old Buckenham High School in Norfolk told pupils and parents on Monday morning that it would be closed due to a member of staff contracting Covid-19.
In the Midlands, teachers at Castle Rock School, which was visited by Boris Johnson last month, and Mellers Primary School in Radford have tested positive. The two schools as well as Blackfordby St Margaret's Primary School in Derbyshire have all closed.
Dozens of other schools across England and Wales have sent pupils into self-isolation, and others have delayed their planned full reopening which began for the first time since March last week.
It comes as Health Secretary Matt Hancock suggested that young people under-25 – particularly those aged 17-21 – have helped propel coronavirus case levels to their highest in three months.
An entire Year 7 year group at Buile Hill Academy in Salford, Greater Manchester, has been sent home until September 18 after one case was confirmed, according to a letter to parents seen by the Manchester Evening News.
The website quotes the letter as saying: "A member of our Year 7 bubble has tested positive for Covid-19.
"The school is working closely with Salford's Public Health England team to reduce the risk of infection for your children and our staff."
Under Government guidance, every member of a school's "bubble" must quarantine for 14 days if one member tests positive.
A total of 30 Year 7 pupils at at Ysgol Bro Edern in Cardiff were sent home for two weeks on Monday after a pupil tested positive, Cardiff Council said on Monday.
Headteacher Iwan Pritchard said the school had contacted all pupils within the student’s class bubble “as quickly as possible” and additional cleaning had taken place.
A class of 21 pupils at at St Gwladys Primary School in Bargoed, Caerphilly, has also been ordered to self-isolate for 14 days after a member of staff tested positive.
The rest of the school will stay open. Caerphilly has seen 78 cases in the past week, the highest number in Wales.
At least eight schools in Wales are believed to have been affected by coronavirus since reopening.
The return to school for some year groups was delayed at Ysgol Bryn Castell in Bridgend, and Ysgol Maesteg after members of staff tested positive for Covid-19.
Meanwhile, in Liverpool as many as 200 pupils and 21 staff members are reportedly self-isolating after outbreaks at eight schools.
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The Samuel Ward Academy, where five teachers tested positive, said in a statement that its closure was a “precautionary measure” and it hoped to reopen on Tuesday.
Headteacher Andy Hunter said: “I will be looking closely at the systems we put in place to try to understand how the transmission occurred and to make sure we do everything possible to limit the chances of the same thing happening again.
“I am very disappointed by this disrupted start to the school term. We have taken very extensive precautions. We were delighted that term had started so well last week and were looking forward to the final two year groups starting (on Monday).
Anyone who has been in close contact with infected staff has been contacted and asked to self-isolate for 14 days, the school said. Further contact tracing will continue and additional pupils and staff may be asked to self-isolate.
Coronavirus cases have also been confirmed at three schools in the area around Middlesbrough.
St Benedict’s RC Primary School in Redcar, St Aidan’s CE Primary School in Hartlepool, and Outwood Academy Ormesby in Middlesbrough have all seen positive cases – but they will remain open.
Meanwhile, single coronavirus cases have been confirmed at three schools in the area around Middlesbrough, but they have not shut.
On Friday the JCB Academy in Rocester, Staffordshire, closed and 100 pupils told to self-isolate after a child contracted the virus.
Geoff Barton, general secretary of the Association of School and College Leaders (ASCL), said it is “impossible” to eliminate entirely the risks of transmission either in school or the wider community.
He added: “It is therefore likely that disruption will continue over the coming weeks and months.
“This shows the necessity for a robust contingency plan in case students are unable to take GCSE and A-level exams next summer or their preparation is significantly disrupted.
“The Government must take action now on such a back-up plan before time runs out.”