The Government’s “big bang” approach to return all pupils in all year groups to class in England is “difficult to understand”, a school leaders’ union has said.

Prime Minister Boris Johnson is expected to tell MPs that pupils in England can go back to school en masse on March 8.

It comes despite calls from unions to adopt a “phased return” of students to classrooms over a period of time – similar to in Wales and Scotland where schools began reopening to the youngest pupils on Monday.

Geoff Barton, general secretary of the Association of School and College Leaders (ASCL), said: “We share the Government’s aim of bringing all children back to the classroom as soon as possible, but we remain concerned about the idea of a ‘big bang’ approach of a full return of pupils all at the same time.

“This is very different from the plans in Scotland, Wales, and Northern Ireland in which a phased return is planned, and it is difficult to understand why the approach in England would go so much further.

Mr Barton added: “It is crucial that the return of all pupils to face-to-face education is safe and sustainable and that we don’t end up prolonging a cycle of disruption.”

Ahead of Mr Johnson’s announcement to the Commons, Paul Whiteman, general secretary of the NAHT school leaders’ union, said the Prime Minister needs to make a “compelling case” if opts for a ‘big bang’ approach to opening schools in England.

He said: “School leaders want nothing more than a safe return to school for all children but, crucially, that return has to be sustainable.

“It is better to be cautious than to risk further closures and yet more disruption to children’s education. A third lockdown for schools would be more devastating than taking our time now.”

But Russell Viner, professor of adolescent health at UCL, said it is “plausible” that schools in England can fully reopen on March 8 while keeping the pandemic under control – if the lockdown is maintained for other parts of society and the rapid vaccine rollout continues.

He said: “Reopening secondaries fully brings the greatest risk for increasing infections, but may bring the greatest benefits for the mental health and life chances of our children.

Schools should be the first part of society to reopen after lockdown. The risks of not reopening schools are high

“Schools should be the first part of society to reopen after lockdown. The risks of not reopening schools are high.”

Prof Viner added: “Schools undoubtedly play a role in transmission of this pandemic, particularly secondary schools. Yet the evidence suggests that transmission can be very much reduced when effective control measures are used in schools.”

The GMB union, which represents school support staff, has called for fully revised risk assessments to be published for a full return to school, and said the Government should fund additional safety measures.

National secretary Rehana Azam said: “School support staff have been in classrooms throughout the pandemic – schools have not been closed. Many school workers are telling us that, without additional protective measures, more space and proper support, a full return won’t work.

“We need full central government funding for enhanced safety measures, extra PPE in primary, early years and Send settings which are now higher risk – and for school workers to be vaccinated as soon as possible.”

John Jolly, chief executive of charity Parentkind, said: “Although many parents are keen for their child to return to the classroom, and many families feel the burden of juggling jobs and other commitments with supporting their child’s remote learning, safety comes first.

“There remains some way to go to reassure parents.

“Before returning their child to the classroom, parents need to feel confident that schools are safe for them, their children and others in their social bubbles when it comes to the spread of the virus.”

In Scotland, children between the ages of four and eight in primaries one to three began returning to school from Monday, along with some senior secondary pupils who need to do practical work.

Children aged between three and seven also started a phased return to school in Wales on Monday, and some vocational learners were back at college for their practical qualifications.

All primary school pupils, as well as older age groups in Years 11 and 13 who have exams, could return from March 15 if Wales’s public health situation continues to improve.

Some primary school pupils in Northern Ireland will return to class on March 8, while secondary pupils in key exam years will return to school on March 22.

Justine Roberts, founder and chief executive of Mumsnet, said: “Parents of primary school children in particular are pretty keen for education to get back to normal as quickly as is safely possible.

“Those who think a staggered return is preferable are worried about transmission, especially among teens and young adults in later school years – but of course this group includes young people facing important exams this year or next, and their parents again are worried about the impact of so much time away from the classroom.”