Primary schools across England will struggle to bring back children in all eligible year groups if parental demand grows in the coming weeks, school leaders have warned.
Geoff Barton, general secretary of the Association of School and College Leaders (ASCL), said some schools already do not have enough capacity to admit all pupils in nursery, Reception, Year 1 and Year 6.
Mr Barton has called on the government to reconsider its ambition to welcome all primary school pupils back to school for a month before the summer break, as he says it is unrealistic.
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His comments follow similar concerns raised by Paul Whiteman, head of school leaders' union NAHT, who warned it will not be possible for schools to accommodate all primary pupils for a month before the end of term, based on the existing advice from the Department for Education (DfE).
The National Governance Association has also written to the education secretary, Gavin Williamson, stating that it is not "practically possible" to achieve a full return to primary schools before the summer holidays.
Earlier this week, a Downing Street spokesman said its plans for all children in primary schools to have a month in class before the summer holidays remained "under review".
But Mr Barton said: "We do not think there is any realistic way in which this could happen. It would not be possible to maintain class sizes of 15 with a full intake of children because there would not be enough space or staff available to do this.
"It would necessitate a completely new set of guidance, and we cannot see how this could be managed safely in the current context. Our view is that the government will need to rethink this part of its plan."
The warning comes as children have been returning to primary school after the government eased lockdown measures across the country.
Schools, colleges and nurseries closed more than 10 weeks ago due to the Covid-19 outbreak, remaining open only for vulnerable pupils and the children of key workers.
But school leaders have warned that they do not have enough space if they adhere to government guidance to limit class sizes to 15, stagger break and lunch times, and encourage fewer interactions.
Emily Proffitt, head of Cooper Perry Primary School near Stafford, said she has some parents on a waiting list this week while she works out how many children she can admit.
Ms Proffitt has opened her doors to around 45 pupils - including students from all four cohorts and key workers' children – but there are more than 100 eligible for a place.
She said she would struggle to accommodate all these children if families wanted them to return.
"The offer to parents at the moment is that they contact us and request a place, but it's not guaranteed. It depends on health and safety. It doesn't sit easily with me because I'd like them all back," she said.
On the government's aim to see all primary school pupils back before the holidays, Ms Proffitt added: "If the government was still insisting on all the safeguarding measures then we cannot accommodate full school. It would be absolutely impossible.
"We can't mitigate risk if the full school is back. If you've got 30 children in one classroom, it's impossible to follow any of the guidelines."
A Department for Education spokesman said: "From this week, many schools have begun welcoming children from Reception, Year 1 and Year 6 back to the classroom as part of a phased and cautious approach.
"To prepare for this, headteachers and school staff have been doing an excellent job including putting protective measures in place and engaging with parents and children.
"We will continue to support schools who haven't yet been able to open more widely to do so as soon as possible."