About 20 per cent of children in the UK are now overweight, while one in nine is obese. Consequently, cases of heart disease and type II diabetes, traditionally adult illnesses, are now being found in children.
In the Wirral, where the number of overweight two to four-year-olds has increased by 10 per cent in the past decade, the local authority is offering all primary pupils healthy meals from September - with chips off the menu.
"Obesity is as serious a health issue as smoking," says Professor Julian Peto, head of epidemiology at the Institute of Cancer Research. "The reduction in life expectancy is the same, seven years. Yet there is little general appreciation of how bad being overweight is for your health.
"A lot of effort is put into educating children about the dangers of smoking. In some ways this is more serious because smoking rates are coming down while obesity is increasing."
Once an overweight child becomes obese the problem usually persists into adulthood and is hard to reverse.
Psychologists say parents are often the last to see their child is overweight. Many do not want to believe it, telling themselves the child is "just big built". Others do nothing for fear of triggering an eating disorder.
Therefore a teacher, says Professor Peto, is ideally placed to be an objective observer. He suggests end-of-year reports should include pupils' body mass index, and a reminder for parents of the long-term health implications.
"Weighing and measuring children once a year would not require specialist equipment," he said.
"Nor would it involve an extra burden on teachers. It would be a good way of generating awareness of the problem. Children and their parents would know where they were on the scale and which way they were moving over several years."
Carolyn Edwards, a clinical psychologist for University College London's Weight Concern charity, agrees that schools have an important part to play in cutting childhood obesity.
But she said: "Parents have an awful lot invested in not seeing the problem. Obese children are often singled out at school for bullying.
"If a child goes home and confides in his parents, they may feel they are colluding with the bullies unless they deny he has a weight problem.
"It is no good unless parents are given proper back-up to introduce sensible measures to help the child, rather than introducing inappropriate crash diets, or ignoring the issue."