Teachers’ unions are calling on the Government to explain how it is safe for children and staff to return to schools in England next month following its publication of scientific evidence.

One union leader accused Boris Johnson of taking a “cavalier attitude” towards children, while another union called evidence from the Government’s scientific advisers “inconclusive”.

The remarks come after the Government published documents from the Scientific Advisory Group for Emergencies (Sage) on Friday.

Sage looked at the scientific modelling for seven different scenarios for school closures, the papers show, but none of the published scenarios looked at the Government’s plan to reopen schools in England to children in Reception, Year 1 and Year 6 from June 1.

One of the documents says the evidence on how likely children are to transmit Covid-19 remains “inconclusive” – but the balance of evidence suggests that it may be lower than in adults.

It remains the case that the National Education Union does not yet think it safe for the wider opening of schools.

Teachers do not appear to be at above average risk compared with other occupations, scientific advisers have suggested, although there is still a chance they can transmit Covid-19.

The publication of the Sage papers – used to inform proposals to reopen schools in England – comes after unions, council leaders and MPs demanded to see the scientific evidence underpinning the decision.

But following their publication, the school leaders’ union NAHT has written to the Education Secretary urging the Government to explain why reopening next month is a “wise thing to do”.

Mary Bousted, joint general secretary of the National Education Union (NEU), said: “We are surprised that the wider opening of schools proposed by Boris Johnson has not been modelled by Sage.

“This points to a cavalier attitude towards the nation’s children. The fact of the matter is that Sage has only weak evidence as to what extent children can transmit the virus to others.”

Dr Bousted added: “It remains the case that the National Education Union does not yet think it safe for the wider opening of schools.”

Patrick Roach, general secretary of the NASUWT, added: “The evidence presented by Sage in terms of justifying the decision by Government to start to reopen schools from June 1 is inconclusive.”

He added that the Sage papers will “only add to teachers’ uncertainty and anxiety”.

“The NASUWT remains of the view that no school should reopen until it can be demonstrated that it is safe to do so,” Dr Roach said.

It comes after a survey from Unison suggests only 2% of school support staff felt reassured by the Government’s recent messaging that schools in England will be safe to reopen from next month.

Wider contextual issues – including whether families have black, Asian and minority ethnic (BAME) members – should be taken into account when assessing the impact of relaxing school closures on transmission, the advice from Sage suggests.

It adds that having different groups of children attend on alternate weeks may be a good way to “stop extensive transmission chains” in schools.

The documents from Sage suggest that younger teachers’ attendance in schools could be prioritised in order to decrease the likelihood of infection for school staff in more vulnerable groups.

But reporters were told during a briefing that Office for National Statistics (ONS) data suggests teachers are not at an “unusual increase in risk” of transmission among than in other professions.

Sources involved said the risk of pupils going back to school and becoming seriously ill with Covid-19 was “very, very, very small” but it’s “not zero”.

They said: “That remains true for every activity outside the house where you might come into contact with people. So it’s totally impossible to say that any change has no risk at all.”

It comes as a separate scientific review of global studies, led by University College London (UCL), has suggested that children and young people could be half as likely to catch coronavirus than adults.

Researchers found those aged under 20 had 56% lower odds of catching Sars-CoV-2, the official name of the coronavirus which causes the disease Covid-19, from an infected person.

With just 10 days to go until schools could reopen more widely, ministers have come under pressure to reconsider their plans to send more children back to primary school next month.

Paul Whiteman, general secretary of the NAHT, said: “Support for a fixed date for school return is vanishing quickly. What is needed now is local flexibility to determine when it is right for schools to open up to more pupils, informed by evidence of what is happening in their own local area.”

Geoff Barton, general secretary of the Association of School and College Leaders, said: “It will quickly become clear to anybody reading the papers that the science is not definitive.”

He added: “The Government must be able to show very clearly that its five tests have been met before it gives the green light to any wider opening from June 1, and it must then be flexible about the timescale and approaches needed to bring in eligible pupils.”

Education Secretary Gavin Williamson said: “The papers published by Sage today show that the phased, cautious return of a limited number of pupils to classrooms has been a carefully considered decision based on the best scientific and medical advice.

“My department continues to engage with the school, college and nursery sectors and I am enormously grateful for all the planning and preparation they are doing, alongside local authorities and academy trusts.

“The welfare of children remains at the very heart of everything we are doing because being able to be back in school will benefit not just their education but also their wellbeing.”