Campaigners are urging the Department for Education to close a loophole to stop schools encouraging parents to opt out of relationship and sex education lessons.
Humanists UK has accused a state faith school of encouraging a mass withdrawal of its pupils from RSE lessons and warned that others could do the same.
It has criticised Lubavitch Senior Girls' School, a Jewish state school in North London, for describing mandatory RSE lessons as "a problem" in an email to parents asking them to opt out.
The government is making RSE lessons compulsory in schools from September 2020 but its guidance says parents can choose to withdraw their children from the sex education element of it.
Debate: MPs discuss whether parents should have opt-out from sex education
An email from Lubavitch Senior Girls', in Stamford Hill, includes a form which it says parents can use to opt-out of RSE lessons.
Sex education guidelines
It tells parents that this subject is not currently taught in school and then adds in bold text: “Please exercise your right to prevent it being taught by responding to this email and saying that you do not want your daughters to receive lessons in RSE.”
The DfE's guidance for RSE lessons states that parents or carers have the right to request that their child be withdrawn from some or all of sex education, which is delivered as part of statutory RSE.
It says that when a parent requests to withdraw their child from compulsory RSE, “good practice is likely to include discussing with parents the benefits of receiving this important education and any detrimental effects that withdrawal might have on the child.”
However, it adds that there is no right to withdraw from relationships education or health education.
Humanists UK’s education campaigns manager, Dr Ruth Wareham, said: “It is extremely worrying that a school is encouraging parents to withdraw their children from these vital lessons so they can completely avoid the duty to provide objective, factually accurate RSE to all pupils.
“We warned the government that schools would try to use this loophole to remove RSE from the curriculum and, unless the DfE takes urgent action to prevent schools from placing undue influence on parents to say they do not want this education for their children, we worry that many other schools may take the same approach as Lubavitch.”
A DfE spokesperson said: "From 2020, relationships and sex education (RSE) will be compulsory for all secondary pupils. This will provide all children, regardless of their gender, the building blocks needed for positive and safe relationships of all kinds.
“Schools will have flexibility to deliver the content in a way that is age-appropriate and sensitive to the needs and religious backgrounds of their pupils.
"As has always been the case, parents will have a right to request that their child is withdrawn from sex education but not relationships education, as we believe the content – such as family, friendship and safety – are important for all children to be taught.”
Earlier this year a petition calling for parents to have the right to opt-out of RSE received more than 118,000 signatures and was debated in Parliament.
The email which Lubavitch Senior Girls' sent to parents states that students around the age of 16 can override a parent’s decision to opt-out.
The form sent to parents says that if a pupil, up to three terms before the age of 16, wishes to receive sex education, the school will provide it.
Lubavitch Senior Girls' has been approached for comment.