Teenagers are not remotely interested in sex. Their idea of a good time is a quiet night in with a wholesome Disney film and some board games. But give them sex education lessons and they immediately trade in the Monopoly board for unfettered debauchery, getting pregnant without even stopping to pass Go.
In an equally improbable scenario, teenagers are sex-crazed animals, their out-of-control libidos feeding off a never-ending stream of hormones. The best teachers can do in such circumstances is to offer contraceptive advice, cross their fingers and hope.
Inevitably, both these versions of reality were presented this week when the Government revealed that levels of pregnancy among under-18s have risen for the first time since 2002. In particular, the pregnancy rate among under-16s has increased by 6.4 per cent in a single year. There were 8.3 conceptions for every 1,000 girls under the age of 16 in 2007, compared with 7.8 in 2006.
Conservative commentators say that this level of pubescent fecundity is to be expected. If schools will go around giving pupils sex education lessons, pupils will want to have sex: it is the procreative equivalent of going on a French exchange visit to practise new-found language skills.
Such arguments continue to have currency, despite their lack of basis in fact. Earlier this year, Janet Elise Rosenbaum of Harvard University published research comparing the sexual behaviour of teenagers in the United States who had taken virginity pledges with that of teenagers who had been taught about birth control and sexually transmitted diseases.
Five years after their virginity vows, 82 per cent of would-be virgins denied having taken a pledge. On average, pledgers and non-pledgers started having sex at the same time, and had similar numbers of sexual partners.
But pledgers were less likely to use birth control, including condoms. Previous studies have also shown that pledgers are more likely to have anal sex at an early age. "Virginity pledges are not a marker for less sexual activity," said Dr Rosenbaum.
Jessica Ringrose, of the Institute of Education, part of the University of London, agrees. "Lots of kids are accessing pornography, are learning about sex through pornography," she said. "When you're surrounded in popular culture by sex, it's outrageous to think they aren't going to know about it if schools aren't teaching it."
Meanwhile, the liberal press has attacked the inconsistency and half- heartedness of sex education on offer in British schools. Teenagers, they say, are inevitably going to have sex. All one can do is educate them about contraception and hope they do it safely.
Such arguments are, to a significant extent, borne out by compelling evidence. Less than 10 years ago, the London borough of Hackney had one of the highest levels of teenage pregnancy in Britain. In 2000, it began to offer comprehensive sex education, from the age of five up. Teenage parents regularly visit Hackney secondaries to talk about their lives. The borough now has one of the lowest teen pregnancy rates in the country.
But these arguments often rest on the assumption that teenage promiscuity is inevitable: the product of a bubbling cocktail of hormones. Yet this assumption sidesteps a deeper issue: that promiscuity is not inevitable at all. For many insecure teens, promiscuous sex is not an expression of intimacy or desire, but a desperate bid to win approval.
Eva, a PSHE teacher from the South East who has helped several pupils with contraception and abortions, points out that many girls believe sex will bring acceptance. A significant proportion of teenage mothers are victims of domestic or sexual abuse.
"These young people haven't had parental love," she said. "Promiscuity is a way of finding love."
And, while comprehensive sex education and free contraception will help to lower pregnancy rates, they are not failsafe solutions. Eva recently helped a sexually active 13-year-old to be prescribed the Pill. But the girl threw her tablets away.
"She said, `If I have a baby, I won't have to go to school,'" Eva said. "Some of them actively want babies. They want to replace love that wasn't readily given by their parents. They think life will carry on as before. When they go out, they'll just take the baby with them. They don't see the bigger picture - they're too young to see the bigger picture."
This view is supported by Hilary Pannack, founder of the sex education charity Straight Talking. She points out that a range of myths surround pregnancy - that the baby will be a doll to dress up, or that it will provide an instant entry into adulthood. Others hope that their boyfriends will stay with them if they have a baby together. And they believe that a baby will provide unconditional love and a new, unproblematic family.
Again, such attitudes are the product of low self-esteem. "Getting pregnant can be a mistake - a drunken encounter, failed contraception, the mood of the moment, whatever," Ms Pannack said. "But becoming a parent is mostly about low self-esteem and a low sense of achievement.
"We need to make young people feel important. We have to give them an educated choice. If you want to be promiscuous, that's your choice. But ask yourself why you're being promiscuous. Is it good for you? What do you want for yourself?"
And pupils need to realise that sex is only a temporary fix: ultimately, it is not enough to compensate for larger feelings of inadequacy.
"They're surrounded by pornified things telling them that having sex is going to make them happy," Dr Ringrose said. "If society teaches girls that the way they're going to get attention and happiness is by being a sexual object - well, duh, those girls are going to have sex.
"Basically, we're just morbidly fascinated by sex. So the whole teenage pregnancy thing becomes a big moral panic where we're policing girls' sexuality. Why aren't we addressing wider issues about self-esteem?"
Sex education, she believes, is not the same as educating pupils for sex. Sex education can teach about contraception, pregnancy and disease, preparing pupils to have physically safe sex. But pupils also need to be prepared for emotionally safe sex.
"The philosophy of PSHE should be spread across every lesson," said Ms Pannack. "We need to talk about relationships, to be firm and guide young people. We need to stay firm in our resolve that sex should be part of a loving relationship. And we need to listen. Let's encourage them. Let's respect them."
But ultimately, Eva points out, the teacher can only provide enough security and guidance to allow teenagers to make their own decisions.
"Part of a teenager's life is experimenting - with cigarettes, drugs, alcohol and sex," she said. "But with cigarettes, drugs and alcohol, you can decide you don't want to get involved. With sex, by the time you decide that, it may already be too late."