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Children are returning to classrooms with twitching thumbs after playing video games and others cannot remember basic spelling and maths.
Some pupils have been out of school for six months and have fallen well behind at a crucial time in their education.
One teacher told KentOnline a Year 5 asked him how to spell 'with' while a Year 6 could not multiply by 10.
"They're quite open about the fact they've done nothing over lockdown," he said.
Steve Chalke, head of Oasis Academies which has a school on Sheppey , has revealed as well as children returning with twitching thumbs some have fallen asleep at their desks after regularly going to bed as late as 5am.
He said: “It is going to take a long time — up to a term — to get routines back into some of these children’s lives.
“They are way behind on ability to focus and concentrate. Their fingers are twitching for something else. Gaming and screen time is addictive.”
Mr Chalke, who also chairs the Sheppey Development Forum, added some pupils are “struggling to cope” with a traditional school routine.
But Tina Lee, who is the principal at Oasis Academy Isle of Sheppey, said she was happy with progress over recent days.
“It has been amazing to see so many happy smiley faces and to see our new year pupils settling in so well,” she said.
“It has been a busy few months getting preparations underway to welcome everyone back, but it is so good to see a bit of normality for our young people.
"All of our students will be kept in ‘bubbles’ with their year groups, we’re all looking forward to an amazing term at Oasis Academy Isle of Sheppey.”
The government were desperate for children to return to school due to fears of how the extended holiday would have impacted them.
But teachers and parents, including head of Simon Langton Boys Ken Moffat, have expressed concerns about the practicalities of social distancing.
In recent days hundreds of pupils have been told to stay at home for another two weeks after members of their year group 'bubbles' tested positive for coronavirus.
It left the start of term in Sittingbourne in chaos after at least four schools reported cases.