You’ve probably heard the expression ‘the camera never lies’,” Nina Hartley says, striding forward for the remote control. “Well, I’ve got news for you: it’s all lies.”

She hits “play”, and an image flashes up on the screen overhead. A plain woman, her skin blemished, hair slightly lank, sits at a mirror. Then, in fast-forward, the makeover begins: her skin is coated in foundation, her hair curled and blowdried. Disembodied hands paint kohl above and below her eyes; mascara extends her lashes. The lighting is adjusted, a wind machine switched on, and the camera begins to click.

The photograph then appears on a computer screen. With a few mouse clicks, the woman’s neck is extended, her face narrowed, eyes enlarged. By the time the resulting image is plastered, larger than life, across a high-street billboard, even the model would struggle to recognise herself.

Ms Hartley switches off the video and reclines into a plastic chair, back straight, legs spread. Images of her in exactly this pose have appeared in the pages of Vogue, as well as in adverts for Marks amp; Spencer and Evans. “When I started modelling, your outfit had to be perfect, your make-up had to be perfect, your hair had to be perfect,” she says. “It took hours; it took all day. Now, it’s snip, snip, snip, and you change it digitally. It’s all just a bit ... it’s all just fake, really.”

For five weeks, Ms Hartley, a model with plus-size agency Excel Models, has been working with Year 9 girls at King Ecgbert School, Sheffield. In a series of one-hour sessions, she hopes to counteract the messages of insecurity and inadequacy that these girls receive daily.

“I was quite tall, quite a big girl, big-boned and all that jazz,” she says. “I thought, ‘Perhaps I should lose a bit of weight, perhaps I’m not good enough.’ Now, I think there’s a balance between being healthy and also having time for relaxation. If you fancy having something a little bit naughty - a bit of chocolate or whatever - then have it. Everything in moderation, as the old saying goes.”

The grandmotherly maxims should not be shocking. But for the pupils listening to Ms Hartley, this is a rare reprieve. Many of the negative messages they receive about their bodies come directly from lessons taught at school.

Schools have moved on since Jamie Oliver drew the battlelines around the turkey twizzler. Food standards introduced by the last government two years ago imposed black-and-white restrictions on school dinners: crisps are out, as are chocolate bars, chocolate-coated biscuits and cereal bars. Pupils are allowed no more than a teaspoonful of ketchup or mayonnaise with their meals; only low-fat milk is allowed.

But some stipulations are somewhat questionable. Growing children need fat for healthy brain development, for example, so eliminating fat from their diet is actively unhealthy.

This approach has also been challenged on philosophical grounds. In June 2008, a study by academics from Loughborough and Cardiff universities argued that school healthy-eating policies have more to do with middle-class moral superiority than medical emergency. The celebration of thinness is an implicit rebuke to the working class, they say, and “endorses the view that bad biology, psychology and habits ... can be apportioned disproportionately to particular categories of the population”.

The study also says the obesity scare exists primarily “to sanction new strategies involving unprecedented levels of intervention, surveillance, monitoring and control, reaching into every aspect of our private and public lives”.

But anxiety over rising obesity levels has prompted a national programme of weighing and measuring every reception and Year 6 pupil in the country.

Perhaps unsurprisingly, this focus on weight and body image can have serious psychological effects. A survey of 80 prepubescent girls for Girlguiding UK in 2007 found that almost all made a link between being slim and being happy, popular and academically successful. Most felt that overweight girls were likely to be miserable, unsuccessful and victims of bullying.

This view is echoed among the girls at King Ecgbert. “If you’re overweight, people think you don’t care about how you look, and you can’t be bothered to make an effort,” says 14-year-old Grace Egan. “It’s about laziness.”

Ms Hartley is on a one-woman crusade to counter these messages. She says the value of healthy eating has been unfairly prioritised and that self-acceptance matters more. She has been leading King Ecgbert pupils in yoga sun salutations, to help them feel at home in their own bodies.

“Hydration, thought processes and breathing are more important than nutrition,” she says. “It’s about creating a balance. Eighty per cent of the time, I try to make the right decisions. But I enjoy that 20 per cent of the time when I fancy a piece of cake, or fancy not going for a run, or whatever I feel like doing.

“Obviously, when I do shows with straight-sized models, everything gets a bit tense at lunchtime. The plus-size models would throw ourselves in, get tucked in. But there would be a few dirty looks coming our way.”

The psychology of the catwalk buffet is equally visible in the school dining room. “The impression is that being thinner means you’re prettier and happier,” says Natalie Coleman, head of PE at King Ecgbert. “But actually, the girls who are thinner are often battling self-esteem issues. People who are thin are not always happy. They’re concerned about how they look to others.”

The link between low weight and low self-esteem is most obvious in anorexic pupils. But Ms Hartley believes there is a broader link between eating habits and emotional need. “There’s a massive grey area where kids have a huge issue around food and use food when they have emotional problems,” she says. “Girls haven’t always been taught how to express themselves, or to deal with anxiety or depression. So they use and abuse food.”

Grace, for example, admits she has “a lot of issues” with the way she looks. “I used to be quite tall, so I feel like I’m a bit fat,” she says. “I tried to stop eating breakfast, but my mum noticed. Then I tried to cut down on chocolate, but that didn’t really work. I’m quite a bad dieter. But I wanted to look like one of those models, definitely.”

Ms Coleman has seen these attitudes spread among pupils. “They see music videos, the Miley Cyruses and the Taylor Swifts, and they’re beautiful and thin,” she says. “The girls try to emulate them but do it the wrong way, by starving themselves and developing unhealthy attitudes to food. It’s shocking the number of girls who skip breakfast, don’t have lunch, then just pick in the evening.”

Their sun salutations complete, Grace and her friends relax on a pile of PE mats. Lizzie Dewick, 14, adjusts her hairband. “It helps that Nina wasn’t one of those stick-thin models,” she says. “If she was, I’d have felt really uncomfortable - I’d have been comparing myself to her a lot. You just find yourself comparing yourself to a lot of people who are thinner than you, and it’s not a nice feeling.

“Normally, I’d be all, ‘Oh, I’d be all right if only I was a bit thinner.’ When I see people in magazines, I definitely think, ‘Why can’t I look like them?’ But Nina’s really pretty, and it just shows that pretty people can come in all shapes and sizes.”

Classmate Katie Day agrees. “She looks healthy,” she says of Nina. “She looks a lot healthier than the really skinny models, so that made me feel better.”

Lizzie interjects: even really skinny models, she says, do not particularly look like models when their image appears on the page. “Did you see how much make-up the model was wearing on that DVD?” she says. “I’ll definitely think twice when I look at a picture now.”

Katie nods. “That woman didn’t even look like a model. She had bad skin, and they made her look totally different: she had a long neck and perfect skin and huge eyes. It’s not normal to have eyes that big.”

Grace puts on her trainers and stands up. “Yeah, but I’ll probably forget that next time I look at a magazine,” she says. “I forget everything.”