Parents have been warned not to take children out of school to avoid isolation during the summer holidays.
The message came from Leicestershire County Council at the same time that a headteacher from Oldham, Greater Manchester, said some kids had been taken away on trips while supposedly in isolation.
The Midlands council sent a letter to parents reminding them that pupils are required to attend school until the holidays.
It said it had received anecdotal evidence some were being kept off to reduce the risk of having to isolate, potentially scuppering planned holidays.
Their warning came amid a big spike in children isolating at home - 375,000 - while just 15,000 tested positive.
Regionally, 3,364 pupils in Leicestershire were self-isolating, with 45 staff members and 235 students having been infected with the virus as of the start of July.
The county council said it believed some families who had planned holidays did not want to risk their children having to isolate if their school bubble is affected.
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The letter to parents read: "Absences not due to Covid symptoms, positive test results or close contact with a case, are treated as unauthorised absences."
According to the BBC Deborah Taylor, the council's member for children and families, said: "We know many families are looking forward to the summer holidays starting but we are aware some have been keeping their children off unnecessarily in advance because they are worried about Covid.
"We want to reassure them schools have been working incredibly hard to remain safe."
Simon Clarkson, from the Leicestershire National Education Union, expressed sympathy for parents who were concerned about their children's safety.
He accused the council of having not invested in ventilation or insist on mask wearing in school to mitigate the risks of coronavirus.
"It would be surprising if the Delta variant was not being transmitted at least in part by in school transmission, especially within bubbles of unvaccinated children," he said.
In Oldham Adam Laskey, headteacher of East Crompton St James' CE Primary School wrote to parents urging them to report rulebreakers to the police, Manchester Evening News reports.
He wrote: "However, it has been disappointing to note that several members of the community have reported seeing children from our school mixing with others in local shops and businesses. Some have even been on holiday. This is unacceptable.
"These children should be isolating, which means they do not leave the house or have any contact with people from outside your household or support bubble, even if they are wearing a mask."
He added: "We encourage any member of the community who is aware of others breaking the law in this respect to report this to the police."
It was announced earlier today that school isolation rules will be markedly different from next year.
All under-18s in England will be exempt from Covid isolation from August 16 if they come into contact with a positive case.
The major change will mean children only have to isolate for 10 days if they test positive for coronavirus themselves.
It came as Education Secretary Gavin Williamson announced the school "bubbles" system would be scrapped from August 16.
He told the Commons: “Keeping children in consistent groups was essential to control the spread of the virus when our population was less vaccinated.
“We recognise that the system of bubbles and isolation is causing disruption to many children’s education. That is why we’ll be ending bubbles and transferring contact tracing to the NHS Test and Trace system for early years settings, schools and colleges.”
