James says he has picked up most of what he knows about sex from the television; the only sex education he had at school was the biological basics at primary school and a few brief talks at the end of year 11.

He's at the Brook sexual health clinic in Brixton for a test, he explains. "I'm here because somebody I slept with had something. I'm 18 and I don't even know the symptoms of it, or what the cure is. It's that bad. All I know is the name." What is it? "Chlamydia."

At Odia's school there was a room where pupils could go and get advice and condoms, and practise putting them on a plastic penis. But there was no obligation to go, and many didn't. Boys were particularly rare attendees. The 17-year-old can't remember having any sex education lessons at secondary school. The pressure to have sex, and to do so without a condom, is intense, she says. "If you make the mistake and say 'yeah, I love you' they use that against you. They say 'if you love me we don't need to use a condom'."

Sexual cyberbullying is a very real concern. "On Facebook you see pictures of naked girls. They've sent it to their boyfriend in confidence and then it's out there," Odia says. She thinks girls need much better education and guidance if they are to avoid this kind of thing: "Remember, we're still young. If we've not got someone to explain what will happen if we do these things, we're not going to know."

Shenee, also 17, agrees. "There was a picture recently of a girl giving oral sex that went round the whole of south London. I think she was a year older than me. We don't ever learn any stuff about relationships. It's always about body parts."

The experiences recounted at the clinic one afternoon last week speak volumes about the lives of young people in a world where sex is everywhere, often in the form of pornography, and the pressures to conform are enormous. These experiences also, the sexual health charity Brook says, back up the demands of a number of leading campaigners, including the Family Planning Association (FPA), that sex and relationships education (SRE) is made a statutory part of the curriculum, ending parents' right to remove their children from lessons.

By law, pupils must learn the biology of reproduction as part of national curriculum science – the only part parents can't take them out of. But there is no statutory requirement to teach them about relationships, sexuality or choices in pregnancy.

The government is currently reviewing personal, social and health education (PSHE), which SRE falls under. Yet ministers have already ruled out making SRE statutory, or ending parents' right of withdrawal. A consultation on the review closes at the end of the month.

It's a missed opportunity, says Simon Blake, Brook's chief executive. "Not making SRE statutory is failing young people. It's failing to listen to them, and if we continue to mess around on this we're leaving them vulnerable to pregnancy, STIs [sexually transmitted infections] and sexual activity they don't want. It's irresponsible. The government has to be brave and grasp the nettle." The UK still has the highest rate of teenage pregnancies in western Europe, and although the figure has now begun to fall, there are fears that cuts to services will undo that progress.

Many schools do a good job, but evidence shows delivery is patchy, Blake says. In a recent survey of more than 2,000 secondary school pupils for Brook, almost half (47%) said SRE doesn't cover what they feel they need to know about sex. A quarter get no SRE in school at all, they said, and a quarter of those who do said the teacher doesn't do a good job of it. An Ofsted report last year found that SRE teaching was no more than satisfactory in a third of schools. "Making it statutory would mean providing a core entitlement for everyone, and would also mean teachers would be trained well enough to be able to deliver it. It's as much about getting people feeling confident about that as it is getting it into the timetable."

And it's relationships that young people want to know about as much as the nitty-gritty of sex, he says: dealing with emotions and real-life dilemmas.

The Labour government had been set to make SRE compulsory, but dropped the plan in order to get its schools bill through parliament before the 2010 election, after failing to win Conservative support for proposals to end parents' right to take their 15-year-olds out of SRE.

According to Ofsted figures, though, less than 0.04% of pupils are removed from lessons. Is parents' right to do so really such an issue? Yes, says Blake, because schools may deliberately water down their SRE offer in order to avoid the administrative problems of some children being taken out of classes. "We know that some schools self-censor because they don't want people to make a fuss," he says.

His assertion is backed by women's rights campaigners, who say allowing schools to determine their own SRE content and letting parents take children out of lessons leads to girls who could be at risk of practices such as female genital mutilation and forced marriage being denied crucial information.

"If you're working at school in an area that has lots of Kurdish people, for instance, and you decide to consult parents, a lot are going to say 'we don't want you to teach about forced marriage because that's a cultural issue'," says Fionnuala Ni Mhurchu, of the Iranian and Kurdish Women's Rights Organisation. "In our experience, a lot of schools will just go along with that. We also hear from organisations we work with about girls being taken out of SRE lessons; they're kept completely in the dark.

"The government uses the language 'parental rights' in the consultation. But while all the human rights instruments recognise the rights of girls not to be subjected to abuse and discrimination, none of them recognise any right by parents to prevent their children from receiving potentially life-saving information."

At Crompton House school, a Church of England secondary in Oldham, the decision was made seven years ago to transform PSHE and its SRE component from an hour-a-week lesson delivered to all pupils by their form tutor on a Monday morning to a standalone subject taught by a much smaller number of staff with specialist training.

"We used to have 40 PSHE tutors all teaching an hour a week," says the school's head of PSHE, Dave Leggett. "You're never going to get 40 skilled PSHE teachers."

Three teachers are now responsible for most of PSHE, and giving the subject a higher profile – including awarding pupils an attainment level for their work – has led to them taking it much more seriously.

In year 9, pupils get a 10-week SRE course that includes three visits from Brook staff. The school has a number of "virtual babies" – dolls that need feeding, burping and changing – that students take home at weekends to learn about the pressures of parenthood.

In Leggett's eight years at the school no one has ever taken a child out of a lesson, and parents are kept fully informed of what is being taught. "We have the babies screaming the place down on open evenings," says Leggett. "Whatever the government's curriculum review says, we will carry on doing it like this, because we think it's important."

In teacher Craig Owen's class, the enthusiasm is evident. Hands shoot up as he quizzes them about a recent session on contraception. What would cause a condom to split? "Didn't put it on right," one boy suggests. "It could have gone off," says a girl in the front row.

Tom, 14, has plenty of questions. "I'm just interested in how the body works and what I could do if I had a girlfriend and she was pregnant," he explains outside the classroom. "I want to know how I could help; I'd want her to feel comfortable with being pregnant."

Nadia Sica, Brook's coordinator for a health and wellbeing programme in Lambeth, south London, funded by the local primary care trust, understands the difficulties schools face in delivering good SRE, which is not part of basic teacher training. "Schools are under a lot of pressure and SRE tends to be at the bottom of the pile because it's not statutory," Sica says. "With all the other things you have to do [as a school], why would you do something you don't have to do?"

The government insists statutory provision is not necessary, saying its policy on dealing with the challenges young people face is not limited to SRE, citing also the Bailey review into the sexualisation of childhood.

"It is not government's role to run families' lives, but it's clear that many parents need more support to bring up their children," a spokesman for the Department for Education says. "We're working closely with industry to make sure that young people are better protected in a rapidly changing technological and commercial world." Plans include putting age restrictions on sexually explicit music videos, covering up sexualised images on the front of magazines and newspapers in shops and making it easier for parents to block adult material on the internet.

"We want to simplify the guidance on sex education to focus on relationships, positive parenting and … sexual consent," the DfE says.

But failing to make those lessons statutory means there's no guarantee schools will take any notice, says Julie Bentley, the chief executive of the FPA.

"We're not pushing for teaching young people lots more about sex. But what we need is good-quality relationships education: teaching them about respect, honesty, trust and self-esteem. Otherwise they'll be entering relationships that are unhealthy, harmful, have violence in them. It's critically important."

• Some young people's names have been changed