When education’s top civil servant visited a secondary school in Nottinghamshire last year he was shocked to find pupils asleep at their desks. Government mandarins deal with the figures, not the facts, of classroom life. But teachers know that more and more children are arriving at school unfit to learn.

“We deal with hunger, tiredness and children who come to school without a pen, a pencil, or even a bag to carry their things in,” says Brian Rossiter, head of Valley school, Worksop. This is the comprehensive that invited David Normington, permanent secretary at the Department for Education and Skills, to experience the realities of school life. “We have daily surges in disruptive behaviour, triggered by what they eat. There’s a big one about an hour after lunch, and a smaller one about 10 in the morning. But some parents simply refuse to co-operate with the school. They just say, ‘It’s none of your business’, when we try to work with them on a problem.”

Rebecca Davies, attendance officer at Cefn Hengoed community school, an 11-16 secondary in a deprived area of Swansea, describes similar problems.

“I deal with children who regularly come to school late, either because there’s no discipline at home, or because mum hasn’t been able to get them out of bed,” she says. “Sometimes there’ll be no food in the house. They are tired. They might have been up all night watching television. Or they might be young carers. For them, school is not a priority.”

But these problems are not confined to hard-core cases, or socially challenging areas. An assistant head at a high-flying secondary school in one of the richest towns in the country cites children who are tired or jumpy, lacking concentration, are on something or hung over, who have jobs before and after school, who lack the right equipment, have problems at home, are unfit, obese, angryI At this point she runs out of fingers and starts round again: children who self-harm, who are out at night, children caring for a parent or siblings at home, children with problems “like you can’t imagine”, and children “who sometimes have to get up and walk out of a class just to hold themselves together”.

“All schools have these children,” says Peter Fielden, head of Roysia middle school, in Royston, a small town in Cambridgeshire where schools and health workers have got together (see box). “The kids are right on the edge, and the most insignificant thing tips them over. We’ve had a hole kicked in a door simply because we asked a pupil to do something. And there’s no guilt or shame afterwards. They and their parents just say that they ‘lost it’. Then there are others who shut down when they can’t cope - the child who goes under the table and won’t come out. Those children are the hardest to reach.”

Needless to say, television gets a share of the blame. John Dunford, general secretary of the Secondary Heads Association, says: “Children live in a television culture. They flick from channel to channel. No one has a sustained conversation over family meals any more. How can you expect children to sit down and concentrate when they come to school?”

But the mushrooming problems of aggression, impulsiveness and poor concentration also have physical roots. One in five 4 to 18-year-olds eats no fruit and vegetables at all, and the average primary school pupil consumes 15 glasses of sugary soft drinks a week, according to a government diet and nutrition survey conducted in 2000. It is widely agreed that the situation has deteriorated since then.

Without healthy food to sustain it, the brain struggles to function properly. “Can a change of diet make a difference? Absolutely,” says Alex Richardson, senior research fellow at Mansfield College and the University Laboratory of Physiology at Oxford University. “Omega 3 fatty acids, for example, are crucial. Without them the brain isn’t going to function as it should.”

One-third of children also now go to school without breakfast. School breakfast clubs help, but nutritionist and health writer Patrick Holford says that what’s on the menu is crucial. A bowl of processed cereal, followed by white toast and jam won’t help children learn, he says. Their brains need an egg, or a kipper, or oat flakes topped with fresh fruit and seeds, to get going in the mornings.

As well as being poorly nourished, many children are also dehydrated.

Researchers at Leeds University have found that the children who do best in school are those who drink eight glasses of water a day, and increasing numbers of schools are beginning to encourage water bottles on desks.

Then there are the related problems of exercise and sleep. A third of all boys and nearly half of all girls get less than 30 minutes exercise a day.

Yet researchers at the University of Exeter, working with local middle school students, have shown that exercise is clearly linked to academic achievement. Children who exercise three times a week do better in their maths and English Sats than those who don’t, and the link holds up even when social differences are taken into account.

And two-thirds do not get enough sleep, according to work done by Loughborough University. A major reason, says Jim Horne, of the university’s sleep research centre, is that children’s bedrooms have become centres of entertainment, not relaxation.

A quarter of all four-year-olds now have a television in their bedroom.

Children who are stimulated by computers, television and mobile phones at night will inevitably show problems the next morning. “When they get tired, they tend to get hyperactive and argumentative,” says Jim Horne. “They are easily distracted and can’t hold their attention for sustained periods.”

The answer, he says, is for parents to set and enforce regular bedtimes, with a wind-down hour beforehand during which all electronic screens are banned. But teachers know that parents are increasingly unwilling or unable to set such boundaries. They know that parents need more education and encouragement, health issues need to be addressed, and vulnerable families need more support in the vital pre-school years. Until the ground rules change, says Brian Rossiter, surveying his Nottinghamshire pupils, schools will just continue to be “the sticking plaster on society”.

Websites: foodcomm.org.uk; foodstandards.gov.uk; wateriscoolinschool.org.uk; wiredforhealth.gov.uk; healthedtrust.com; sportengland.org