Tossing the salad. Shag bands. That "two girls and a cup" video. None the wiser? Well, you're not a teenager, then. Your personality isn't an existential mix of bravado, insecurity, lovability and Lynx; you don't screech as an everyday method of communication; you aren't overcome with the desire to show the world your individuality by playing grime really loud from your mobile while waiting for the number 159.

If you're not a teenager, then that also means you're unlikely to know the sexual significance of a brightly coloured jelly bracelet. And I'm guessing (though I may be wrong) that you've never received unasked-for pornography on your phone first thing in the morning.

Me neither, though I do know several teenagers who have. Because I've been making a documentary for Radio 4 on sex, porn and young people. It started innocently enough, with the producer and me investigating the recent trend for "shag bands". This was a short-lived craze among teens (and younger kids) for wearing armfuls of rubber bracelets. Each colour meant a different sexual act: if another kid broke your band, you "had" to do whatever act that was signified by the colour, from kissing to anal sex.

The teenagers we spoke to about shag bands were lofty about them; it was clear that few regarded them as serious. When I asked if they knew of any of their friends who had actually acted as the snapped shag band required, only one said yes: "But they were drunk." Several mentioned that their parents had found out what the bracelets meant and stopped them from wearing them; many schools had banned shag bands too, including primary schools.

So we spoke to an eight-year-old and 10-year-old as well, who knew what shag bands were. "There were a couple of boys who were giving them out as free presents," remembered one, "and saying you had to sex someone if you broke it… nobody told the teachers." (They also informed us that some of the latest playground insults are "you chocolate-flavoured condom" or "you rainbow-coloured condom", both of which will definitely be used in my next argument.)

If I'm honest, none of this information particularly worried me. Teenagers will always be obsessed with some ridiculous clothing trend and they will always be interested – and only partially informed – about sex. Only partially, because they read and hear about sex ages before they actually do it (at least, you hope so). It's where they get their information that is worrying. Because – despite governmental efforts to improve sex education, to allow teachers to acknowledge emotions and relationships as well as which bit goes where – that place is, of course, the internet.

"I just went into Google and typed in 'cock' and 'boobs' and stuff," recalled one lad of his early online interests. Boobs, which, in my day, used to be a calculator word (58008, upside down), is now a gateway to an online breast assault. Cock leads to more hardcore results. Porn is just a mouse click away from almost every teenager, no matter whether you set up parental controls or not.

And the difference between the porn that teenagers see these days and the grubby mags passed around school in the 80s and 90s is vast. We spoke to a psychologist who said that video has a stronger effect on the watcher than still pictures, because what you're seeing is more lifelike. Second, contemporary porn is different – ever more waxed, tanned, surgically enhanced – which adds to the pressure for young people to have hot bodies.

Perhaps that still doesn't really worry you. After all, if you don't want to see sexual images, kid, don't go looking for them. Unfortunately, as anyone who's used Twitter over the past few weeks knows, you don't have to be actively searching for porn for it to arrive uninvited into your life. Many of our boy interviewees mentioned the "two girls and a cup" video: a film of two sexy ladies relieving themselves into the same receptacle.

This spread from mobile to mobile among teenagers – whether the receiver wanted to see the film or not – and led to many anxious 12- and 13-year-old boys wondering just what exactly girls would want to do once they did eventually end up having sex. Another girl told us that she kept getting pop-ups and links to porn sites while innocently MSN-ing a friend. She was 14 at the time and, now 18, still feels upset: "It was horrible – you can't get rid of those images."

But what I found most shocking are the real-life films. Teenagers have sex and use their phones to video each other as they do. And then, post-break-up, or even while they're still dating, one of them – usually the boy – bluetooths the film to everyone he knows. And beyond. This happens more than we might care to admit. I didn't have to dig very far to find one 15-year-old girl who'd suffered in this way, which, obviously, can lead to awful humiliation and distress.

All this before we even get to the dubious joys of stripping for strangers for Chatroulette, of older men using Facebook to groom young people, of inexperienced teenagers trying out some of the more advanced techniques they've seen in porn…

So what should we be doing about all this? Anything? Nothing? It's the classic clash between adult responsibility and teenage kicks, between letting young adults discover what life is like for themselves and helping them negotiate the tricky bits. One headmaster told us that parents should actively stalk their teenager's online life, just to know what their kid is up to, so they can step in if, for example, they suspect that a Facebook friend is nothing of the sort.

What would be better still would be to arm children with enough information so that they're aware of such situations developing and can tell someone who can help. Kent County Council has appointed an e-safety officer, Rebecca Avery, who trains teachers and other school professionals to have a more open approach towards kids' use of technology. She says that, often, young people don't want to tell adults if they've seen something that makes them uncomfortable because they assume the grown-up will take away their mobile or computer, leaving them socially bereft. Because – of course – they're not allowed out to meet their friends in real life, because it's not safe…

And I wonder whether sex education classes shouldn't take more account of modern life. Knowing what a sperm and egg get up to in the privacy of their own boudoir is all very well, but understanding that if you let yourself be videoed having sex that film can be used against you is perhaps more useful. The fact is that some of today's teenagers are zooming straight past spin the bottle to specialist interest. And we aren't helping them learn how to deal with it.

Sex, Porn and Teenagers is broadcast on Radio 4, Monday 4 October 2010, 8pm