Good news is a rare commodity, so here's a cause for celebration. Teenage pregnancy has fallen at an astonishing rate in recent years, "dropped like a stone", said one surprised national statistician. Why? It's no accident, but a prime example of determined social policy, driven nationally and implemented locally with ringfenced funds, good training, joined-up departments and constant checking to see where and why it was working best. Here's a success for what the right calls "social engineering".

At the time many said it couldn't be done: governments can't change deep, intractable cultural mores. Some leftwing social theorists dismissed it as fixing symptoms, not economic causes: these girls were making a rational choice in their dire circumstances. Louder voices on the moral right rejected sex education, opposed friendly local contraception clinics, TV campaigns, school nurses telling girls how to get help and chemists selling morning-after pills.

But the programme ploughed ahead – and it worked, with advisers at its heart from Brook, the service most experienced in sex advice for the young. Few know about this success because good news gets scant reporting. Besides, this counterfactual flies in the face of belief, rubbing against the grain of every "to hell in a handcart" Mail wail, where the young are always worse than their parents in culture, education and, above all, their frightening sexual habits.

Here are the facts: since 1998, the proportion of pregnancies in under-18s has fallen by a remarkable 34%, to 30.9 per thousand. Until then the number had been rising since the late 1960s. But here's what people wrongly think: Ipsos Mori says people think 25 times more under-16s get pregnant than really do. Oddly, the 15- to 24-year-olds themselves grossly overestimate the rate. They think 40% of their own generation of under-18s get pregnant – which would mean 12 in a classroom of 30. Most people (80%) think teen pregnancy is still rising.

The Conservatives ran a grossly misleading campaign at the last election, following Iain Duncan Smith's fear-inducing Breakdown Britain reports. Moral panic about everything was their theme, figures often wildly wrong. An election document called Labour's Two Nations claimed that a staggering 54% of 15- to 17-year-olds in the most deprived areas were getting pregnant. The real figure was 5.4%. When caught out, they said it was an errant decimal point – but that figure was repeated three times without anyone at Tory head office stopping to wonder whether it could possibly be right. That suggested deep ignorance of life in the large parts of Britain they don't represent, distant "bad lands" of their worst imaginings. At that time Chris Grayling was called out by the UK Statistics Authority for his misleading use of figures to try to claim violent crime was rising, when it had been falling for years – and still is. But ignorance is the right's friend, bogus figures bolstering wrong public estimates of benefit fraud (24%; real figure 0.7%) or numbers of black and Asian people, (a third, real number 11%). When people talk of "single mothers" they imagine girls, like the "young ladies" Peter Lilley mocked in the horrible "little list" he once sang to the Conservative party conference, about teenagers getting pregnant to get a council flat. In real life, the average age of single mothers is 38.

Here's another interesting fact: of the declining numbers of teens who do conceive, many more choose abortion. Nor are the outrageous young behaving any worse than their parents, according to a recent report in the Lancet. The survey of sexual attitudes and lifestyles by the London School of Hygiene and others is a vital 10-yearly study that was personally cancelled by Margaret Thatcher, who thought it prurient, until it was rescued by the Wellcome Trust. It finds teenagers are not having more sex or sex at a younger age than they were 20 years ago, with no change in the average age of first sex, still 16. Here's an odd fact: in 2010, the proportion of conceptions rose for women in their 20s, 30s and 40s – but not for the under-20s.

The fall in teen pregnancy is no accident or whim, but almost entirely due to a programme that should be a model for social policy, backed by evidence and monitored for success so it could be altered as it progressed to learn from what worked best. The 1999 teenage pregnancy strategy had the advantage of an outcome measure that can't be massaged by proponents, nor denied by doubters, its results set out in Office for National Statistics conception and birth numbers.

Back in the optimistic days at the start of the last Labour government, when 18 study groups set out to track and eliminate causes and effects of what was somewhat euphemistically called "social exclusion", the teenage pregnancy unit was set up. A modest investment of £25m has led to the probable non-birth of 60,000 babies to very young mothers, children whose lives on every measure would have been harder than average. Local clinics, nurses and well-trained teaching in sex, relationships, anti-bullying and resisting peer pressure had an immediate effect. Results were markedly different between places that cut pregnancies dramatically – Lambeth, Hackney and Hull – and similar areas that didn't, attributable to where there was good sex education, with good contraception services, and where too little was done.

Will it last? Teen pregnancy rose steeply in the 1980s with raised poverty and unemployment. The figures lag by more than a year, but so far they are holding up well. Alison Hadley, now of Bedford University, who ran the teen pregnancy unit until it was shut down, says the best local champions are still in place, and schools that embraced good sex education have it embedded in their culture. But she worries at the huge cut in youth services, a crucial part of reaching young people most at risk outside school. The National Youth Agency says the outlook is grim: no longer ringfenced, councils are cutting teen pregnancy funding by 19% this year. Labour failed to make sex education a statutory part of the school curriculum and Michael Gove has no interest, removing all this stuff from his department. Will the new chaotic NHS commissioning give priority to school nurses and good contraception services for the young?

The lesson from this programme is that extraordinary things can be done, people can change, but it takes both strong central conviction and good local enthusiasts to see off the doubters.