Concerns have been raised about a sex education programme for teenagers which includes information about oral sex and the practicalities of gay sex.
Official advice says children need to be confident about discussing sex
The A Pause scheme was developed at the University of Exeter and is now running in about 150 secondary schools, funded by health and education authorities around England.
It was the first such scheme in Europe to show a reduction in rates of sexual activity among young people - with the UK having the highest teenage pregnancy rate.
But some are concerned that it is too explicit.
Doncaster teacher Lynda Brine, writing in the Times Educational Supplement, says an A Pause training course she attended did not make children aware that sexual intercourse below the age of consent - 16 - was illegal.
A course such as this gives children information they do not or should not know
Her main concern centred on a part of the course about how teachers should respond to "frequently asked questions".
"Examples included when a 14-year-old girl asks: 'What does semen taste like?' Or a 15-year-old boy: 'How do gay men have sex, and is it possible for a man and woman to do it the same way?'
"I ask myself why children of this age ask such things."
Ms Brine is a single mother of two and an advanced skills science teacher, who says she does not speak from any "moral high ground".
"A course such as this gives children information they do not or should not know," she argues - and says she can play no part in it.
The A Pause programme says it is not "an abstinence project, but an enabling programme supporting young people in their decision to delay intercourse until a time when they are less likely to regret it".
Its manager, John Rees, said the course did make it clear to children that sex under 16 was against the law.
The programme was about getting 14 and 15 year olds to think about "stages of intimacy" that did not involve penetrative sex, with its risks of pregnancy and infection.
"It's about saying to them, 'You can hold hands, you can kiss and cuddle', it may even get as far as something like oral sex or even mutual masturbation.
"We talk rather coyly about exploring above the waist and below the waist with a view, not to encouraging them to this level, but to frank discussion about whether these things are appropriate."
Family values campaigner Robert Whelan, of Family and Youth Concern, said the scheme had good elements.
"I can see what they are trying to do. Everybody wants to cut down on teenage pregnancy," he said.
But the course went too far.
He approved of its advice that holding hands was an acceptable way for teenagers to show each other affection without appearing "un-cool".
"I wish they had left it there. 'Don't go on, go down' is not very helpful either."
The Department for Education said it did not fund the A Pause programme.
OFFICIAL SEX AND RELATIONSHIPS GUIDANCE
Children need to develop confidence in discussing sex
Teachers can use a range of strategies:
establish ground rules
de-personalise situations using role play
use discussion and project learning
"We trust heads and teachers to make sensible decisions about sex education, just as with other aspects of education," a spokesperson said.
Official guidance says sex and relationship education - as it is now formally known - "is about understanding the importance of marriage for family life; stable and loving relationships, respect, love and care".
"It is also about the teaching of sex, sexuality and sexual health. It is not about the promotion of sexual orientation or sexual activity."
It suggests each class should negotiate a set of ground rules about what is appropriate.
"Many teachers are concerned about responding to unexpected questions or comments from pupils in a whole-class situation.
"If a teacher doesn't know the answer to a question, it is important to acknowledge this, and to suggest that the pupil or teacher or both together research the question later."