Education Secretary Gavin Williamson has sought to reassure parents and teachers worried about government plans to get some children back to school on 1 June.
Mr Williamson, speaking at the daily Downing Street briefing on Saturday, said the plan to allow very young school children to restart at the start of June and others to start later was based on the "best scientific advice" and put children at the heart of their decision.
It comes after teaching unions said they were willing to work with the government to begin to re-open schools in England – provided it could be done safely without the risk coronavirus could flare up again.
On Friday, after a meeting with the government's scientific advisers, union representatives said they had been left with more questions than answers, with one union leader describing the scientific evidence as "flimsy at best".
They were backed by the doctors' union – the British Medical Association – which said schools should not reopen until the numbers of coronavirus cases were "much lower". Scotland, Wales and Northern Ireland are also yet to set a timetable for a return to school.
The education secretary said at the briefing: "I know lots of you will be worried about sending your children to school. Every one of us wants the very best for our children and I know how stressful this time has been for families across the country.
"I want to reassure you that this approach is based on the best scientific advice, with children at the very heart of everything we do."
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Mr Williamson added that his department has set up a "hierarchy of controls", insisting that people stay at home if they have coronavirus symptoms and that proper hand-washing, hygiene and cleaning routines would be put in place in schools.
He said the reopening would be done in "a very, very cautious and phased way".
He praised teachers for "going above and beyond the call of duty" for continuing to teach children of key workers, as well as making sure children at home were given the resources they need.
"You have simply been outstanding and we are so grateful for what you've done," he said.
"We have been quite clear all along that we'd only start inviting more children when our five key tests have been met. That position has not changed nor will it."
But he added: "We can now start the planning for very limited return to school for some pupils potentially as early as next month."
He said mixing was a "key element" of what needs to be done, but that was "why we have gone in this initial stage of much-reduced sizes down to a maximum of 15".
"We have looked all across Europe to see how this best works," he added. "We have seen some good examples in nations such as Denmark where it seems to have worked very well.