Lunch boxes stuffed with convenience foods are on the increase. For as the cost of school meals rise, more parents opt for packed lunches. And, say some heads, because so many parents work, large numbers of children are eating crisps, chocolates and sweets in the middle of the day.
But child health experts think some diet-conscious heads are going too far - celery and carrots might be all right for overweight adults but not for growing children who need high-calorie foods.
One head who has strong views on packed lunches is Geraldine Tausig at Annemount nursery and pre-preparatory school in north London. She has banned sweet and savoury snacks from lunch boxes.
In a letter to parents, she explained: "For the coming week we are discussing diet and healthy eating. The discussion of contents of lunch boxes fits into this and I am explaining to children why the Annemount school policy insists that there are no crisps, sweets, biscuits, chocolate and other 'jealous-making' goodies in lunch boxes.
"The principal reason is that children need good energy in the middle of the day as well as nutritious foods to help their growing bodies. I have explained to the children that it is fine to eat sweet foods later in the afternoon when it is nearer to bedtime and easier to brush their teeth."
One of Ms Tausig's parents is Dr Dee Dawson, medical director of the Rhodes Farm Clinic in London which cares for 32 child anorexics. Dr Dawson says: "I wonder how many other headteachers are sowing the seeds of the idea of good and bad food to such young children. It is amateur advice coming from people who know nothing about nutrition, child development and growth."
Dr Dawson, whose youngest patient is six years old, thinks young, energetic children need nuts, crisps, cakes and biscuits to provide calories. She is appalled that children as young as four are being told there are good and bad foods, and has ignored Ms Tausig's advice. Her six-year-old daughter, Alex, takes cakes and crisps, as well as sandwiches to Annemount.
Drinks are also a vexed question. Jenny Evans, head at The Havers infant and nursery school in Bishop's Stortford, Hertfordshire, has banned fruit juice and fizzy drinks at lunch-time. Only water is allowed.
She said: "I see an awful lot of rubbish lunches come to school which cost more than a school dinner. You wouldn't believe the excuses parents come up with for not wanting their children to drink water."
Schools are not picnic areas, complains Mrs Evans, who has also banned chocolate. Half-finished fizzy drinks or fruit juices, returned to lunch boxes so parents can see how much their children have drunk, leak on to floors and attract ants. And lunch boxes have become so big that the school has had to buy special trolleys to store them.
Dr Jackie Stordy, formerly a children's health specialist at Surrey University and now director of nutrition research at the food supplements company, Efamol, has no problem with a non-interventionist policy.
She said: "Children need plenty of calories to grow and it is unusual if calories come without anything else. Even in crisps there are other nutrients as well. I think children should have a variety of foods in their lunch boxes and that variety should not be determined by school policies because parents know what the child is eating for the evening meal and breakfast."