The National Education Union (NEU), the largest teaching union in the UK, has written to the prime minister calling on him to explain his decision not to shut schools across the country.
Teachers are increasingly asking why the government is not closing schools in the UK, especially now plans are under way to suspend mass gatherings, the letter from the NEU says.
It comes after Ireland announced on Thursday that schools would close for a fortnight in the wake of the outbreak.
But Mr Johnson has said closures could do “more harm than good”.
The letter, from the NEU’s general secretaries, says: “It is very important that we understand what the increased rate of infection is for staff and parents if schools remain open, including obviously for those with underlying health conditions themselves, or for those they care for.”
It adds: “We all want to limit and delay the spread of the coronavirus and we do think medical advice and expert scientific advice is important in this regard.
“But it is clear that such advice can have uncertainties, and that it could be crafted to target a variety of measures.
“We now see that you may take legal powers to force schools to remain open even when heads and teachers think there is good reason to close.
“In those circumstances we, as the leaders of the largest education union, believe that it is right for us to ask you for fuller disclosure of the models you have looked at, and to understand which measures you are targeting.
“We do not have the medical expertise to know what the transmissibility is between children and staff in close quarters in classrooms – but your scientists will have made assumptions about that, together with some view of the certainty of those figures.”
Speaking to the media at the Association of School and College Leaders (ASCL) conference on Friday, education secretary Gavin Williamson said schools are “one of the safest places” for children to be.
In a speech to hundreds of heads, Mr Williamson said: “The chief medical officer has said the impact of closing schools on children’s education will be substantial, but the benefit to public health would not be.
“The government is particularly mindful of the strain on public services like the NHS that would be caused by key workers having to stay home to look after their children as a result of school closures.”
The letter, from NEU bosses Mary Bousted and Kevin Courtney, adds: “We would suggest that parents and schools would be able to work together to find solutions to that, and we would like to know if you have any modelling of such societal responses.
“Most of all we think that education staff deserve to have access to modelling of the projected spread of the virus and the projected number of fatalities in a wide variety of scenarios including in scenarios where schools are closed for different periods of time.”