All lower classes should simply repeat the school year that began last autumn when things return to normal.
That is one suggestion grandetante but what do we do about all next year's Reception children who are due to start in September? Do we make the age of entry to school rising six? I can't see parents being very happy to sit around for another 12 months before their child starts.
Some children are almost a year older than others in the same year group anyway. I don't think they need to repeat a year. Any half-decent primary school teacher can adapt to children at different stages.
There is absolutely no reason set in stone anywhere (apart from the national curriculum) that children at a certain age should be able to do x, y or z. I think parents have become obsessed with those progress books they are given when a child is born stating that a toddler of 12 months should be able to do certain things. Real children aren't like that.
Your last paragraph is so true, different children, different sexes, different backgrounds and individuals' different ways of working all contribute to being able to do things at very varied ages.
The awful SAT's were set up to ensure that schools were enabling children to progress at a "normal" rate and were there to test the schools, not the children - but the result has been an awful pointless competitive trial for children and teachers rather than schools.
I'm convinced that children won't be harmed by "missing" a year of school. They will (hopefully) be learning different things at home and can learn what they need to when they return to school.