Head teachers have been threatened with legal action if they take an active part in the Covid-vaccination programme.
Pressure group Lawyers for Liberty warned school staff could be held liable if families objections are not listened to.
Health workers - and not school staff - will vaccinate 12- to 15-year-olds.
One in eight in are already vaccinated because they, or someone they live with, are clinically vulnerable.
Pupils in schools already receive jabs against other conditions every year, for which consent is gained from parents, guardians and carers, as part of national NHS-run immunisation programmes.
Covid vaccines being used have been through the same testing and approval process - the process was simply completed faster because of the worldwide focus on tackling the pandemic.
And millions of over-12s have received the vaccine around the world in countries already offering the jab to teenagers.
Health Secretary Sajid Javid said the decision to immunise against Covid had been taken after "expert advice" from the UK's top scientists and vaccination experts.
Details are due to be published shortly but it is widely expected school staff will have no role in the immunisation programme, despite the jabs being offered in school settings.
One letter from Lawyers for Liberty, which has been widely circulated to schools and seen by BBC News, talks about "exercising their parental responsibility during the decision-making process".
It adds: "If a parent communicates to you that their child will not to be included in the vaccination programme or does not provide consent, then that decision must be respected, without any further consequences for the child, including direct or indirect discrimination or coercion.
"Failure to do so may result in possible legal claims against you personally and for your school."
Association of School and College Leaders general secretary Geoff Barton said: "Many of our members have been receiving letters from various pressure groups threatening schools and colleges with legal action if they take part in any Covid-vaccination programme.
"This is extremely unhelpful and we would ask those involved in this correspondence to stop attempting to exert pressure on schools and colleges."
National Association of Head Teachers general secretary Paul Whiteman said the letters were "misguided", stressing the decision about immunising children was the government's and school staff would not be vaccinating children .
Clear guidance on the immunisation programme was needed without delay, he said.
"Now that a decision has been made, it is essential that the government immediately confirms that the process surrounding vaccinations will be run and overseen entirely by the appropriate medical teams,
"Where parents have questions, including about important matters such as consent, these must be handled by those same medical teams.
"There must be no delay in confirming this.
"Otherwise, school leaders will be put in an impossible position of facing questions to which they simply do not have the answers."
Mr Barton added: "The vaccinations will be administered by healthcare staff and any disagreements over the question of consent between children and parents, which is likely to be extremely rare in practice, will be resolved by healthcare staff.