Teachers have suggested spraying pupils with disinfectant at the school gates amid fears of how to enforce social distancing rules among young children.
Primary school students could start returning to school in England from June 1 - but the majority of secondary school pupils will not attend class until September at the earliest.
Boris Johnson said the start of June was the earliest possible date to consider the phased reopening of schools, beginning with some of the youngest pupils in reception classes, year one and year six.
But even this staggered approach caused alarm, with the leader of the largest teaching union calling it "reckless".
Blasting the plans, Dr Mary Bousted, joint general secretary of the NEU, suggested we should follow schools in China and South Korea where 'children stand outside the school gates and are sprayed front and back with disinfectant'.
And another union raised concerns about how social distancing can be managed with younger children.
Dr Mary Bousted, joint general secretary of the NEU, said: "We think that the announcement by the Government that schools may reopen from June 1 with reception and years one and six is nothing short of reckless.
" Coronavirus continues to ravage communities in the UK and the rate of Covid-19 infection is still far too great for the wider opening of our schools."
According to the Sun, Dr Bousted added: "In China, children stand outside the school gates and are sprayed front and back with disinfectant, their shoes are sprayed, they wash their hands with sanitiser, they must take off their mask and replace it with a new one, and their temperature is taken remotely."
Asked if a similar regime should be introduced here, she said: "Yes. They're doing that in China and South Korea and they have a minuscule number of new cases."
The general secretary of a teachers' union said the profession has "very serious concerns" about children returning to school on June 1.
Patrick Roach, of NASUWT, told BBC Breakfast: "The fact of the matter is the Government has announced a date but hasn't come forward with a plan about how schools will ensure that they're safe for pupils and safe for staff to be in from June 1.
"And the Prime Minister said that it would be madness to risk a second spike in relation to transmission of the virus.
"Well the profession has got very serious concerns about that announcement of June 1, whether indeed it is possible to achieve it, but also how to achieve that in a way which is safe for pupils and staff."
He said there is strong evidence schools are lacking personal protective equipment (PPE), adding: "If you're dealing with five and six-year-olds and 11-year-olds, how to ensure stringent social distancing in that context is a big challenge and Government simply haven't answered that challenge.
"And finally, just in terms of risk assessments, parents will want to know that schools are going to be hygienic, they're going to be safe for their children to be in.
"And we still don't have any clear standards about what safe cleaning routines would be like within a school context and we need to have that."
