Secondary schools in the UK have been plunged into the centre of the row over Covid vaccines for 12- to 15-year-olds, with anti-vaxxers at school gates and a headteacher threatened with legal action by one of his own governors.

Letters circulated by campaign groups and parents are accusing schools of sanctioning “medical experimentation” if they allow the Covid vaccination programme for 12- to 15-year-olds to go ahead.

The headteacher of a secondary school in the south-east has been sent one of the pro forma letters, which was signed by a member of the school’s own governing body. It said she would hold the head personally liable if children were given Covid jabs without parental consent.

On Thursday, members of the campaign group Outreach Worldwide gathered in north London to hand out leaflets and talk to pupils at home time as part of its “informed consent campaign”, which aims to “educate and empower teens and encourage resistance”.

Heads are expecting anti-vax groups to ramp up action in coming weeks as schools begin to distribute consent forms to families, followed by a staggered start to the vaccinations.

Critics of the programme claim parents’ wishes will be ignored because of the “Gillick competence”, a legal ruling which states that if a young person has the “intelligence, competence and understanding” to appreciate what is involved in a medical treatment, they can receive it whether parents agree to it or not.

Guidance published on Friday by the Department for Education (DfE) said that in cases where a teenager wanted the jab but their parent had not given consent, healthcare professionals would make every effort to contact the family before they proceeded, but that the parent “cannot overrule the decision of a Gillick competent child”.

The guidance also confirmed that the local School Aged Immunisation Service (SAIS), not schools, would be legally responsible for the delivery of the vaccine.

Geoff Barton, general secretary of the Association of School and College Leaders, warned that headteachers were feeling “inundated and pretty intimidated” by the anti-vax campaigns and feared being “caught in the crossfire”.

“There is a head of steam behind it that needs to be addressed in a very forthright manner by the DfE,” he said. “Headteachers are not going to be overruling parents. Our job is to open a sports hall and send a letter out; that is the end of our responsibility. But heads are concerned that in instances where there are disagreements in families about vaccinations, schools will somehow get caught in the crossfire. We are saying to our members you are not in any way expected to get caught up in it.”

Concerns have also been raised that the “febrile atmosphere” surrounding the debate could put parents off agreeing to their children being vaccinated.

The government is reportedly aiming for 75% of the age group to have the jab, a figure which some headteachers described as unrealistic and which is much higher than the 56% of children in secondary school who received the flu vaccine last autumn.

Parent polls also suggest there will be a lower take-up. A Parent Ping survey last year found that just 52% of mothers would allow their children to be vaccinated, but that 76% of fathers were in favour, suggesting there could be some disagreements within households.

“I think initially, the feeling was that there would be quite a high level of consent because these young people have suffered so much with loss of schooling and their parents have seen that,” said Barton. “But I think that through the way that this has unravelled – with all these counter-arguments – it will have unnerved some of those parents and that could well drive the numbers down.”

At the beginning of September, the UK’s vaccine advisory body refused to approve vaccinating healthy children aged 12 to 15 on health grounds alone. The Joint Committee on Vaccination and Immunisation (JCVI) said children were at such a low risk from the virus that jabs would offer only a marginal benefit.

However the UK’s four chief medical officers, who have the final say, have recommended vaccines for 12- to 15-year-olds on “public health grounds” because it is “likely vaccination will help reduce transmission of Covid-19 in schools”.

Steve Bell, chief executive of the the Painsley Catholic Academy Group of Schools, in Staffordshire, said he was unconcerned about the quasi-legal letter he had received from an anti-vax group.

He said: “It’s a tactic to try and put some pressure on but the way I look at it is that we are in the middle of a devastating pandemic and we just want our children to be in school because they’ve missed out on so much. We are already, at the start of this term, facing a large number of Covid cases. The statistics say that having the vaccine will reduce transmission and serious illness. That is great for the kids because it means statistically, they will be more likely to be able to come to school.”

A Department for Education spokesperson said: “It is never acceptable for anyone to pressurise or intimidate pupils, teachers or the wider school community.

“In the same way as vaccinations for pupils in schools take place every year, the School Aged Immunisation Service will manage and be legally responsible for the delivery of the Covid vaccine to 12- to 15-year-olds.

“Consent from parents or carers will always be sought for this age group and it is up to children and their families if they want to accept the offer of the vaccine. No one should be stigmatised for their decision.”