Non-uniform day was supposed to be a day of liberty, of escape from constraints and controls. Instead, girls at the Bristol school were growing increasingly stressed. “I’m so fat,” one said. “Everyone will be looking at me. Everyone will be able to see how fat I am,” said another.

They were seven years old.

Chris Calland, a teacher who specialises in addressing body image issues, was called in to the school to help tackle these concerns. “Many children, much, much younger than we thought, are being impacted by these issues,” she says. “I don’t think we realise that it’s seeping in so early on.”

A recent parliamentary report on body image offers numerous stories to corroborate this. There was the six-year-old girl who repeatedly asked her father: “Daddy, do I look fat?”; the 12-year-old girl who dreaded going to school because she felt that her body was being scrutinised by her peers; and the revelation that one in four seven-year-old girls has tried to lose weight at least once. Boys, too, are not immune: a third of boys between the ages of 8 and 12 say that they are on a diet.

Now the equalities minister, Lynne Featherstone, has launched the Body Image Parent Pack, a 12-page guide aimed at tackling youngsters’ body fears. It cautions that any comments about weight and body shape - as well as airbrushed images of celebrities - are damaging children’s self-esteem. The guide quotes a study by Girlguiding UK, which found that 75 per cent of 11- to 21-year-old girls go on diets to look more attractive.

Much is heard about the obesity crisis. Right now it’s National Childhood Obesity Week, which aims to raise awareness “of the dangers of being above a healthy weight during childhood”.

The intentions are undoubtedly good. But they pave a road few would want children to follow. “There is such an emphasis on obesity nowadays,” says Mary George, of eating disorder support organisation B-eat. “It can have a real effect on young people. They become obsessed with how much fat is going into their bodies, how much fat is in the food they are eating. There can be too much attention paid to that word. Children are very, very vulnerable, and can be very easily swayed.”

No one disputes that good health is an admirable goal. But, the parliamentary report emphasises, it is incorrect to assume that “overweight” and “unhealthy” are interchangeable terms. It states: “Many overweight people live longer and healthier lives than people classed as a `normal’ weight . Around one-third of obese people are metabolically healthy.”

Since 2006, every primary pupil has been weighed twice, once in reception and once in Year 6, to determine whether they are a healthy weight. This is done by calculating their body mass index (BMI), a figure based on weight and height measurements.

BMI was pioneered in the 1830s by a Belgian polymath, Adolphe Quetelet, who believed that people who deviated from the average physically were more likely to be social deviants as well. Excessive weight, he believed, was an indicator of moral laxity. The measure was later popularised by insurance agents, keen for any excuse to increase clients’ premiums.

But BMI was always intended to map demographic trends, rather than as a pass-or-fail test for individuals. Yet today, 11-year-olds are taking home letters that award them body-image grades: “normal”, “overweight” or “obese”.

“On its own, BMI is a pretty blunt instrument,” says Jo Swinson, the MP who chaired the body image parliamentary group. “Medical professionals will use a range of measures: waist circumference, diet, activity levels.”

Children’s weight, especially towards the end of primary school, also fluctuates considerably. Pubertal girls, in particular, often lay down additional fat prior to a growth spurt. Labelling them “overweight” only leads to low self-esteem. “Children associate fatness with being stupid, being smelly,” says Calland. “They begin to obsess about whether they are fat.”

In a 2007 survey carried out by Girlguiding UK, almost all of the 80 girls questioned made a connection between being slim and being happy, popular and academically successful. Similar numbers felt that overweight girls were likely to be miserable, unsuccessful and victims of bullying.

Fixated on food

Meanwhile, excessive focus on healthy eating can lead to a distinctly unhealthy fixation on food. Calland talks about a 10-year-old boy who, following a school healthy eating programme, became obsessed with the contents of his lunch box. “He only wanted to eat fruit and veg, and worried about eating fat,” she says. In fact, fat is essential for children’s growth and brain development.

“I’ve seen children as young as 6 talk about `sinful food’,” Calland adds. “It’s so unhelpful for children to start labelling food: `this is good food’ and `this is bad food’. You don’t want them to be getting this relationship to food at an early age. It can have massive implications as they get older.”

National Childhood Obesity Week organisers plan to publish the results of a Let’s Talk About Weight survey, examining the ways parents broach the topic with their children. But talking about weight is exactly what we should not be doing with children, according to Calland.

“The obesity message shouldn’t be going to kids,” she says. “They just hear the message `fat is bad’. We say to children that it’s what’s on the inside that counts, but then we communicate a different set of values: what you look like is what matters.”

She and a colleague have created a series of lesson plans for primary pupils, which avoid talking about weight, food or diets. Instead, they focus on diversity, health and self-esteem.

Swinson, too, would like children to be taught that weight, like height and skin colour, is simply a manifestation of individuality. “If children of any age are paranoid or anxious about the way their body looks, then they are far less likely to engage in exercise,” she says. “Encouraging people to focus on a healthy lifestyle is much better than blinkered focus on a number on the scales.”

National Childhood Obesity Week runs until Sunday (www.mendprogramme.orgaboutusmendin2012ncow); the Body Image Parent Pack can be downloaded from www.mediasmart.org.ukparents-pack.php

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