They scare the other passengers — meek, tired and irritated — and use a secret language punctuated with words such as “sket” and “nang” and “buff” and “bredren”, which mean whore, cool, good-looking and mate, in that order. I look at them longingly, because I know they are cool, they are the real “It” girls and I want to be 15 again, and beautiful and shouty and mixed-race, like the prettiest ones are, except I can’t because I am a middle-aged white Hausfrau, well past it and, more importantly, past “It” — the art of being effortlessly cool. I can’t get it or be it, but I can still hunt for it.

At Islington Green School, a mixed comprehensive in North London, I am in a room of 15 and 16-year-olds who are going to tell me what cool is and what cool is not. I am subscribing to the trickle-up theory of cool, which is that cool starts from the street and works its way up to the focus groups and marketing men and big brands.

I have brought along a copy of the NME, an enormously inappropriate and past-it ice-breaker, as it turns out. Nobody in this room knows who any of the people on the cover are, because they are not hip-hop and R&B stars, who are the cool ones, but “rock freaks” who are not cool at all. Somebody immediately zooms in on Billy Joe from Green Day. “He’s wearing make-up, that’s not cool — it’s gay.” Pete Doherty is dismissed as a “waste man” and the rest are dismissed as “Don’t know ’im. If I don’t know him, he can’t be cool.”

The first thing they tell me is that cool is not a cool word any more, that they all say “nang”, but even that is kind of old, and it is better to say “shabby”, “gunny”, “grimy”. And when things are not cool they say “That’s cold” or “I’m in despair”, and when people are not cool they call them waste girls or waste boys. If they feel they have been disrespected they don’t say “dissed” any more but say that they have been “boyed”, as in looked down upon and called “boy”. But not the girls. To slag them off you call them, as one girl reels off with great relish, “Oh, a sket, a waste girl, an apple, a what-up girl, a tramp, a ho.”

Nahid, 16, says: “I am cool and I am a grade A student. I am a shepherd, not the sheep. Big up Bengalis . . .” Smoking is “sooo not cool”. They all say this, but Nahid the most poetically: “If you want to be cool you have to live, and if you smoke you will be dead, and that is not cool, to be dead.”

Mobile phones are cool, says one boy, only if you have lots of girls’ numbers on them.

Surprisingly, the consensus is that the right gear, the right trainers, clothes, etc, is not as important as grown-ups think. “People don’t care about that no more,” says Rahel. “Cool is something that is unique, being different.” She thinks the coolest job would be to be a lawyer, not for the big money but “because you get to defend people who need help”. Other cool jobs are being a footballer, being your own boss and, this from the coolest-looking dude in the shop, “selling medicine”.

Nobody thinks black is cooler than white (“That’s racism, man, innit.”) but they do say it sounds dumb when black people use white slang or white people use black slang. The latter I know. But what is white slang? “Oh, all that EastEnders stuff like safe, sweet and mate. It sounds stupid.”

In the playground afterwards, they talk sweetly about their husbands and wives. “Where has my husband got to?” They explain it is just messing around, they pretend to be married to each other. “Like boyfriend and girlfriend?” I ask, reasonably. They look horrified. “Eww. No! No!” It is blazingly clear to all, and to me, that I just don’t get it.

A few weeks later I am in a coffee shop with 13-year-old Hazel Lee, a girl I have known since she was a toddler, who has effortlessly morphed into a walking emblem of street cool.

“Mayfair are the best cigarettes because they are like the best property on the Monopoly board,” she says through a mouthful of croissant. “But they are also £2.07 compared to B&H which are £2.35.” She adds hastily that she does not smoke herself, but “most cool people do”.

Clothes are important, she says, with her crowd. “You can’t wear trainers that aren’t Nike or adidas or Academic. If you wear trainers with no name they are called space forces. And if you have like an old cheap mobile phone, it’s called a brick.” But all that stuff takes money. How do you get the money? “You just do, or if you don’t, it’s OK if you make a joke about it, that you can’t afford something.”