Teachers' leaders have warned that suggesting a date for schools to begin reopening more widely risks creating "false hope".
The country's biggest teaching union, the NEU, made its comments after prime minister Boris Johnson announced earlier today that he hoped schools would be able to reopen to more pupils from 8 March.
Boris Johnson: We hope to reopen schools from 8 March
Gavin Williamson: Teachers to get two weeks' notice of school reopenings
"If we achieve our target of vaccinating everyone in the foremost vulnerable groups with their first dose by 15 February, and every passing day sees more progress towards that goal, then those groups [will] have developed immunity from the virus about three weeks later," Mr Johnson told the House of Commons earlier today.
"That is...we hope it will therefore be safe to begin the reopening of schools from Monday 8 March, with other economic and social restrictions being removed thereafter, as and when the data permits," he added.
Coronavirus: Reopening schools 'is a balancing act'
Teachers' leaders have said they recognise that the decision to reopen schools more widely is a "balancing act" but that to set a specific date risks creating "false hope". They accused the prime minister of falling into "misplaced optimism".
Dr Mary Bousted, joint general secretary of the NEU teachers' union, said: "We all want schools to open, but, like the prime minister, we want them to open when it is safe to do so. This has to be done sustainably and safely.
"We agree with Boris Johnson that this is a balancing act. He has a duty to assess the easing of lockdown according to the progress and effects of vaccination, a reduction in cases and the various other criteria he has set out.
"But in setting out a potential date of 8 March, falling once again into his characteristic and too often misplaced optimism, he is pre-empting a decision that will have to be made in mid-February at the very earliest.
"If we come out too early, we will end up in lockdown again," she added.
"Hinging his argument for schools according to the first four vaccine groups developing immunity by 8 March is not enough in itself. This may protect the elderly and most vulnerable adults in the population, but it does not protect parents. It fails completely to recognise the role schools have played in community transmission.
"The prime minister has already forgotten what he told the nation at the beginning of this lockdown, that schools are a 'vector for transmission'.
"It would have been fine to have set out a roadmap, but to suggest a date at this stage runs the risk of creating false hope. The prime minister may now be immune to the embarrassment of U-turns, but school leaders, teachers and support staff, not to mention families and students, are utterly exhausted by them."
School leaders also called for an end to "confusion" over the use of lateral-flow rapid coronavirus tests and cautioned that there were too many "unknowns" for the 8 March date to be confirmed.
Geoff Barton, general secretary of the Association of School and College Leaders, said: “What is crucial is that the government ensures that full reopening is done in a way which is safe and sustainable, and which inspires the confidence of education staff and the public.
“It must straighten out the confusion over the use of rapid-turnaround coronavirus tests, review its guidance on safety measures for schools to make sure these are fit for purpose, and set out a timetable for the vaccination of education staff."
And Paul Whiteman, general secretary of the NAHT school leaders' union, said that Mr Johnson's statement made it clear that "there are too many unknowns, such as the effectiveness of the vaccine and the pace at which infections are falling" to put the 8 March date firmly in the diary.
“It is also important that the teaching workforce is prioritised for vaccinations," he added.
"This would give confidence, as well as providing a better chance that, once lockdown measures are lifted, children’s education is less likely to continue to be disrupted by staff absence and illness.”