A headteachers’ union is calling for older pupils to return to school first as part of a phased approach to reopening schools in England, amid warnings that students preparing for exams may need to repeat the whole year because of the impact of lost learning.

The Association of School and College Leaders (ASCL) said the pupils who have most to gain from getting back to school are those in years 10 and 12 who are in the middle of GCSE and A-level courses, and those in the final year of primary, preparing to move to secondary.

The approach contrasts with Denmark, which became the first country in Europe to reopen its schools last week, when under-12s were the first to return. Younger children are less able to study on their own and having them back in school allows parents to return to work.

Speculation surrounding when and how schools in England will safely reopen has become increasingly fraught as the lockdown has persisted, with sharp disagreement among ministers and divergent views among teaching unions.

Geoff Barton, the general secretary of the ASCL, said the earliest schools might realistically reopen would be in June after half-term. “It can only happen when supported by the science, and there will need to be a lead-in time of several weeks to ensure it is carefully planned.

“It is then going to be necessary to maintain social distancing in schools as much as possible. It is likely that we will need to reintroduce certain year groups in the first instance rather than fully reopening schools to all pupils.

“This could be particularly beneficial for pupils in year 10 and year 12 because they are studying GCSE and A-level courses respectively, and for pupils in year 6, who are due to go to secondary school in September.”

Pressure to reopen is mounting amid growing concern, particularly for disadvantaged students who will be hardest hit. The former head of Ofsted Sir Michael Wilshaw, speaking on BBC Radio 4’s World at One, warned that some pupils preparing for exams might have lost out so much that they would have to repeat the whole year.

Other teaching unions, which are worried about schools having to reopen prematurely because of fears over the economic impact of parents having children at home, have denounced speculation about return dates and focused purely on the public health risks to pupils, school workers and the wider community.

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The NASUWT wrote to the education secretary, Gavin Williamson, this week setting out five conditions for reopening, including access to personal protective equipment where required, ranging from soap to gloves, aprons and in some cases face masks, as well as guarantees on adequate staffing and physical distancing advice.

The NASUWT general secretary, Patrick Roach, also warned that teachers should not be expected to clean. “The NASUWT would not expect teachers to be asked to undertake cleaning tasks or to be expected to undertake them to the necessary standards to protect the health of pupils and the workforce.”

The National Education Union, which is the biggest with 450,000 members, has insisted the science should decide when schools reopen. Its petition to delay reopening until safe to do so has gained more than 160,000 signatures.

Paul Whiteman, the general secretary of the school leaders’ union NAHT, said: “Schools should only reopen when the scientific evidence is clear that it is safe to do so. Safe for pupils, safe for staff, safe for parents.”

Since 23 March only vulnerable children and the children of key workers have been attending school in person, while teachers have remained working on distance learning and support for pupils at home. Attendance in the emergency school provision has been much lower than expected because parents are keeping their children at home.

A Department for Education spokesperson said Williamson had not set a date for schools to reopen. “They will remain closed, except for children of critical workers and the most vulnerable children, until the scientific advice changes and we have met the five tests set out by government to beat this virus.

“We will work in close consultation with the sector to consider how best to reopen schools, nurseries and colleges when the time is right so that parents, teachers and children have sufficient notice to plan and prepare.”