The plan to have every primary school pupil back in the classroom for at least a month before the summer holidays is under review, the Government has said.
Downing Street had previously stated that it was their “ambition” that primary schools would be fully open by the end of June to allow children four weeks of lessons before they break up again for the summer.
But this is now likely to be shelved, The Telegraph understands, after ministers were warned that it would be “logistically impossible” to achieve.
Geoff Barton, general secretary at the Association of School and College Leaders, said that getting all primary school pupils back before the summer was “unworkable”.
“If class sizes can be 15 at a maximum and you are genuinely going to have every child in, you will need double the class sizes and double the teachers,” he said.
“Officials were quick to realise that unless you can suddenly rustle up double the classrooms and double the teachers, it is logistically impossible.”
Schools across the country opened their doors to pupils in Reception, Year One and Year Six on Monday, but up to half a million pupils were forced to stay at home after over 50 councils defied the Government’s plans.
Last month, the Department for Education released detailed guidance advising schools on how to prepare for reopening, which stipulated that classes should be split in half with a 15-pupil limit.
Each group should be assigned one teacher, and children should spend their break times and lunch with the other children in their class to ensure that they are only mixing with one “bubble”, the document said.
On May 10, the Prime Minister announced the Government’s blueprint on how to ease lockdown, which included telling schools to prepare for Reception, Year One and Year Six pupils to return from June 1.
He said that secondary school pupils in Year 10 and Year 12, who are in the middle of their GCSE and A-level courses, would also be allowed some time in the classroom before the summer.
The Government’s detailed recovery strategy document stated that the “ambition” was for primary schools to be open for all year groups by mid-June so children can have a month of lessons before the summer holidays.
Mr Barton said the fact that Mr Johnson’s address to the nation on May 24 made no mention of this was a “striking” omission from the speech and indicated to him that this had been dropped. “We made a case about the logistics and I think it was well received,” he said.
The National Association of Headteachers (NAHT), the main union representing primary school leaders, is advising its members to ignore plans for a wider reopening by the end of June, saying this was “unrealistic”.
The National Governance Association, which represents school governors, is the latest organisation to oppose the plans. Emma Knights, the organisation’s chief executive, warned that it is not "practically possible" to get all primary school children back by the end of the month.
Around 90 per cent of primary schools will reopen either this week or next, according to a poll by the NAHT. The majority (78 per cent) intend to open with a smaller number of year groups than the amount recommended by the Government, with 12 per cent intending to open completely in line with ministers’ proposals.
The remaining ten per cent of schools say they will not allow any pupils - other than the most vulnerable and the children of key workers – to return in the next fortnight.
No secondary school pupils will return to their classrooms until June 15, when students in years 10 and 12 will return on a limited basis.
It is not clear yet when other children will return, but any attempt to bring them back to classrooms over the summer holiday may be resisted by teaching unions.
Gavin Williamson, the education secretary, said: “I recognise there may continue to be some nervousness among families, but I want to reassure parents – and those working in schools and nurseries – that the welfare of children and staff continues to be at the heart of all of our considerations.”