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Howard Fisher, head of St George's Church of England Primary School in Sheerness, said he'd rather see his pupils retake a year than risk their lives by being forced back to school too soon.
The letter was shared on the school's Facebook page after Boris Johnson's announcement on Sunday that some classes would be phased back into school from 1 June.
The letter - which has since been shared 15,000 times - reads: "We have no plans sent from the government before the announcement last night. No doubt they will arrive this week suggesting social distancing, less pupils in school, splitting them up, staggered lunches and drop offs, etc.
"I can be truthful here and categorically tell you there is no such thing as social distancing in a school; it does not exist and would never exist.
"The reason childhood illnesses spread in a school is surprise, surprise, we are all in contact with each other. I can put two children in opposite classrooms and they will still get chicken pox because that's how it is in a school.
"This virus we are led to believe is a super spreader."
The Government has since outlined a number of measures it plans to implement to enable schools to reopen safely, but Mr Fisher went on to warn that these measures he anticipated may not be enough to prevent the spread of Covid-19.
The letter continued: "Do I want the staff and the children back; of course. Are you at the end of your tether at home; of course you are.
"However I am not going to sit here and write to you to say we can achieve social distancing in a school. We can always make things safer, we could perhaps reduce slightly the risk, but as soon as you open the school as far as my many years can tell you, the risk will be there.
"So that leaves us all in a quandary doesn't it? How long can we go on like this for? Unfortunately I heard no answers last [Sunday] night, I heard no solutions.
"I heard that it is OK to put our youngest children back in a school and our oldest. I heard nothing that would put my mind at rest.
"There will be some of you that say, 'let's just get on with it,' I respect that, but get on with what?
"There is not a reliable test, a vaccine, an idea about what to do next, there is just the possibility that things will be OK; that's all we have at the moment and 'OK' is not good enough when it comes to the precious gift that is your child.
"Believe me, I would rather any child repeats a year than go back too soon and have to lose a child; why is this not in the national debate; because it will cost money!"
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