Teachers' leaders have insisted they were "right" to call for school closures because of Covid last March.
The NEU teaching union's joint-general secretaries also said today that education secretary Gavin Williamson was "blind to the gravity and danger of the situation" at the beginning of this year and, "like King Canute", he "stood on the shores of ignorance" telling teachers that it was safe to return.
If the government had instead adopted the NEU's education recovery plan, transmission rates in schools would have been cut and pupils would have spent less time away from the classroom, Mary Bousted said.
Revealed: The science behind school closures
She was speaking at the annual NEU conference alongside fellow joint-general secretary Kevin Courtney.
Mr Courtney also reported a "huge surge" in the union's membership.
"We are 35,000 stronger than this time last year," he said.
"And we want to welcome all those new members to the union – be they teachers, support staff or school leaders in England, Wales or Northern Ireland."
Reflecting on the NEU's policy during the pandemic, Mr Courtney said: "Time and time again we have been proved right. We were right to argue for schools and colleges to close to most pupils last March. We were right then to say that clinically extremely vulnerable (CEV) and clinically vulnerable (CV) staff should make their contribution working from home.
"Right to ask other staff to be in schools and colleges, working on a rota to support those key worker and vulnerable children.
"We were right to argue, along with Sage [the Scientific Advisory Group for Emergencies], that there should have been a two-week autumn half term as a circuit breaker and then with secondary schools operating on a rota basis. If the government had listened there would have been [fewer] deaths and less disruption to education."
He added: "When the government ignored us, we were right to argue that schools should have been included in the November lockdown to suppress transmission rates from schools into the community.
"We were right to argue that schools in Greenwich should be allowed to close the week before Christmas because of the new variant and soaring transmission rates."
And looking back to the beginning of this year, Dr Bousted said: "We were right that primary schools should not reopen on 4 January in the middle of a pandemic, with infection and hospitalisation rates soaring, and with a new variant which was known to be up to 50 per cent more transmissible."
She added: "Gavin Williamson had decreed that primary schools would reopen come what may. Blind to the gravity and danger of the situation, he kept on carrying on.
"Knowing that it was unsafe to fully open schools, like King Canute he stood on the shores of ignorance telling education professionals that it was safe to go back. It was not."
Dr Bousted said that, if the government had adopted the NEU's recovery plan, "transmission rates in schools would have been reduced and pupils would have spent less time out of school home-learning".