Unions representing school and college staff and headteachers said they were “increasingly concerned” the government could opt for this plan, as they raised concerns it could risk a surge in coronavirus cases and drag out disruption faced by students.
The prime minister is expected to lay out plans when he sets out a “roadmap” for easing England’s lockdown next week.
In a joint statement on Friday, the group of unions said they were “committed to bringing all children and young people back into the classroom as soon as possible”.
“However, it is counterproductive if there is a danger of causing another surge in the virus, and the potential for a further period of lockdown,” they said.
They urged Mr Johnson to commit to welcome children back on 8 March “only if the scientific evidence is absolutely clear that this is safe” and if so, to only go ahead with a phased return.
“We are increasingly concerned that the government is minded to order a full return of all pupils on Monday 8 March in England. This would seem a reckless course of action,” the coalition said.
“It could trigger another spike in Covid infections, prolong the disruption of education, and risk throwing away the hard-won progress made in suppressing the virus over the course of the latest lockdown.”
The coalition – which includes the Association of School and College Leaders (ASCL), National Education Union (NEU), Unison and Unite – said science over the impact of schools in the overall rate of transmission was “uncertain”.
“What we do know is that the full reopening of schools will bring nearly 10 million pupils and staff into circulation in England – close to one fifth of the population,” they said. “This is not a small easing of lockdown restrictions. It is a massive step.
This means a “cautious approach” to wider opening of schools and colleges is needed, which should be “phrased over a period of time”, the nine organisations – which also included GMB, National Association of Head Teachers (NAHT) and Sixth Form Colleges Association (SFCA) – said.
On Monday, Mr Johnson said no decisions had been made on whether year groups across schools in England will return together, or whether primaries and secondaries could be staggered.
He said last month that schools in England would remain online to most pupils until at least 8 March, as they have been since early January.
Total attendance in schools has been consistently higher during this current lockdown compared to March to May last year, when schools were also shut to all but key worker and vulnerable children.
The number of children attending primary schools last week was six times higher than during the nation’s initial spring lockdown, according to government data.