After bagging up the day’s rubbish as normal, one particular evening the school cleaners didn’t throw it all in the skip. Instead, they left it where the children could find it, and the following morning, the pupils came in and emptied the bags all over the floor.

Fortunately, they had covered the hall floor with plastic sheeting first. And with the rubbish all spread out, they got to work. They put on their plastic gloves and started to sift through it, separating paper from fruit peel, cardboard from discarded food. “It was a pretty nasty job, but it was a lot of fun as well,” says Julie Tallant, head of the school’s eco team.

Once it had all been sorted into its separate piles, each pile was rebagged. Food waste, both raw and cooked, made up three whole sacks in a school where only about 60 of the 200 children had school meals.

But the school, Dodworth St John the Baptist Primary in Barnsley, is far from exceptional. Schools produce around 20kg of food waste per primary pupil every year, and 6.5kg per secondary pupil, according to research carried out by Waste amp; Resources Action Programme (Wrap), which campaigns to reduce waste and promote recycling.

While around a fifth (22 per cent) of all household waste is made up of food, it makes up a much greater proportion in schools. Almost half (46 per cent) of all rubbish thrown out by primary schools is food, making it by far the largest single source of waste. Even in secondary schools, food makes up 31 per cent of all rubbish.

“We were surprised by the amount of food waste being produced by schools,” says Lindsay Silver, Wrap’s schools programme manager. “Although all our research shows that schools think recycling is important, food waste is just not on their radar at the moment. It isn’t something they think about.”

Schools are not the only offenders. Around a third of all the food we buy in the UK ends up in the bin. Much of this sits languishing in our cupboards and fridges until it is well past its sell-by date. As a result, as a nation we end up throwing out an astonishing 8.3 million tonnes of food a year, around 138kg a person, equivalent to a 21-stone man. Cutting our food waste to zero would save an average household around 50 a month. “People always say they don’t waste any food, but as a nation we are wasting phenomenal amounts - we just don’t realise it,” says Mrs Silver.

Following the publication of its initial study into school waste last year, Wrap is carrying out a qualitative study. In a large-scale equivalent of tipping the bags on to the floor at Dodworth St John’s, researchers are working in 39 schools to provide a breakdown of food waste. The aim is to discover what is being thrown away, and where changes need to be made.

This new research, expected to be published in the spring, should identify what proportion of the waste comes from over-ordering raw supplies, how much is the result of serving food the children don’t like, or serving too-large portions, or whether it is a combination of all of these.

Wrap’s Love Food Hate Waste campaign has identified a number of causes of household waste, including lack of planning in food shopping, poor knowledge of how food should be stored and confusion over the difference between “use by” and “best before” dates. The findings of this latest study will be used to produce a similar template for schools.

Advice to schools to reduce waste will be drawn up after the study, depending on where the major problems lie, says Mrs Silver. “We want to find out if the waste is coming from preparation in the canteen, whether it is being cooked but not served, or served but not eaten, or whether it is from packed lunches,” she says. “We need to understand more about the problem to know what to do about it.

“It is not necessarily over-ordering, but there is more choice in schools now and potentially there is a connection between more choice and waste.”

The initial research has already thrown up some interesting findings. Larger quantities of raw - as opposed to cooked - food waste accounts for much of the difference between primary and secondary schools in overall food waste. A significant part of that raw waste from primaries is whole or nearly whole pieces of fruit.

Children in reception and Years 1 and 2 in England have been entitled to one free piece of fruit a day since 2004, under a 36 million Department of Health programme. It seems, however, that much of this fruit either goes straight into the bin or has only a brief visit to a child’s mouth.

Barely touched fruit and veg was conspicuous by its presence in the waste tipped out on the floor at Dodworth St John the Baptist, says Ms Tallant. “A lot of it was bananas, tomatoes, carrots and oranges.”

In response, the school started cutting the fruit into chunks. Pieces of orange or half a banana proved much more palatable than a whole one. “They are much happier taking small pieces from a plate,” says Ms Tallant. If it still didn’t get eaten, it was given to the older children.

The school also took control of its kitchens from the local authority and set about changing the food on offer to try to make it more appealing. A survey asked the children what they wanted to eat, and the result is a menu dominated by freshly cooked food, such as fish pie, homemade pizza, spaghetti with freshly made bolognese sauce and homemade pasties. “Since introducing the new menu, we don’t tend to have a lot of food waste,” says Ms Tallant.

Squares of chocolate were on offer as an incentive to children to clear their plates, and parents were given tips on reducing waste from packed lunches. Any raw leftovers are put in the school’s two compost bins, for eventual use in its garden and vegetable patch.

A term after their initial audit, the pupils once again tipped a day’s rubbish on to the floor. Instead of three sacks of food waste, the school now produced only one. Take-up of school dinners has also risen, from about 30 to 50 per cent of pupils.

But the balance between meeting nutritional standards and reducing waste is a fine one, says Lesley Bowyer. If the food is unappealing, then it will end up being thrown out, however healthy it is, and sausages, pizza and chips can appear much more enticing to children than limp salads and vegetables. “Food waste is a good indicator of what children like,” she says.

Mrs Bowyer, headteacher at Carr Green Primary School in Brighouse in Calderdale, West Yorkshire, says the school developed new menus after surveying pupils. Freshly made salads are now as popular as sausages, and the waste bucket no longer has to be emptied halfway through a lunch sitting. There are still blips, however: curry did not prove as popular as anticipated and resulted in a waste surge.

As well as the financial cost, food waste also has an environmental cost. The 100,000 tonnes of food thrown out by schools every year is responsible for 422,000 tonnes of carbon dioxide, the equivalent of 130,000 cars.

An estimated 20 per cent of the UK’s greenhouse gases are associated with food, including production, distribution and storage. Wrap believes that cutting out wasted food would prevent the emission of at least 15 million tonnes of carbon dioxide. Most waste food is buried in landfill sites. As it decomposes, it gives off methane gas, a greenhouse gas 25 times more powerful than carbon dioxide.

Carr Green took delivery of a composter to recycle raw fruit and vegetable waste - the danger of attracting rats and other vermin means cooked waste needs to be treated before it can be recycled - with the compost then used on the school allotments. The composter was moved to another school while Carr Green underwent a major building refurbishment, but Mrs Bowyer says it proved successful while it was in operation.

“It didn’t reduce the amount of waste, but it did mean we were sending less off-site,” she says. There was, however, a downside: the smell of rotting vegetables was overpowering, although it did prove rather alluring for flies.

Some local authorities collect food waste - cooked and raw - but turning it into compost tackles only one side of the problem. “Collecting food waste isn’t reducing food waste,” says Emma Jones, senior education project officer for environmental charity Eco-Schools.

She says the priority is to cut down on the waste in the first place, by ordering the right amounts of food and making sure schools are serving food the children will eat.

Where schools do use compost bins, she says it is important to turn the compost regularly to discourage rats from nesting. Lining the bin with chicken wire can also be a good deterrent.

Even though it does not have its own kitchen and has cooked food brought in from outside, St Polycarp’s Catholic Primary School has still managed to cut food waste. The school, in Farnham, Surrey, phones through the exact number of children who will be having lunch every morning, once the registers have been taken.

“Straight away, that reduces the waste of providing food for children who aren’t here,” says Lucy Waugh, eco-committee co-ordinator and a Year 3 teacher.

The school’s dinner ladies also brought in stickers - featuring an empty plate with a knife and fork - to reward children who finished their meals. “A lot of waste was from children not eating their meals, maybe because they weren’t keen on a particular thing, but they love the stickers and they are really proud when they get one,” says Ms Waugh.

As at Dodworth, uneaten fruit from the school’s fruit and vegetable scheme made up a sizeable proportion of food waste. Now the leftover fruit is offered to older pupils, or kept for the next day. Anything remaining at the end of the week goes for composting.

But an arms-length relationship between school and caterer can prove an obstacle to cutting waste, according to Vassia Paloumbi, sustainable schools programme manager for the environmental charity Waste Watch. She says schools need to work closely with their caterers to ensure the food is what the children want and the portions are appropriate.

Along with the food itself, by-products increase its environmental cost, particularly where the meals are not cooked on the premises. “I have been to schools where most of the waste is packaging from deliveries,” she says.

An analysis of food waste at Ringmer Community College found a large proportion was either raw vegetables, often peelings from food preparation, or salads used in sandwiches.

“The majority of our waste is card and paper, but we wanted to look at food waste as well,” says Stephen Green, environmental co-ordinator at the 800-pupil secondary in East Sussex.

Primary schools may produce around three times as much food waste per pupil as secondaries, but anecdotal evidence suggests that primaries are also further along the road in recycling food. Ringmer is unusual among secondary schools in composting food. It has also found an outlet for used cooking oil, normally classed as hazardous waste. The oil is collected and used to power buses in Brighton.

Overall, the school now recycles around 110 tonnes of waste a year, saving 3,000 annually on waste collection charges. Instead of filling four large skips a week, Ringmer now fills three every two weeks.

Schools are waking up to the need to reduce food waste, but taking action is often harder in secondaries, says Dr Paloumbi at Waste Watch. “Because primary schools are smaller, it is a lot easier to get the children onboard,” she says.

Leftover meat, vegetable peelings and salad made up much of the discarded food at Colonel Frank Seely School. Its audit took in the kitchens, food technology areas and the staffroom.

“We were already doing a lot to recycle plastic bottles and cans and paper, and we wanted to see what else we were wasting,” says Trina Ridgill, business manager at the secondary school in Calverton, Nottinghamshire.

Mixing cooked and uncooked food makes it unsuitable for recycling, so the school installed separate bins to take different types of leftovers. “It is about teaching the children to be proactive,” says Mrs Ridgill. “Just because they don’t want it, it doesn’t mean it can’t serve a purpose.”

At Canon Burrows CofE Primary School, much of last year’s Christmas dinner ended up in the waste bucket. And this even included the sandwiches from the Christmas party.

“There wasn’t any halal meat and there wasn’t a great variety of vegetarian food,” says Andy Clark, eco co-ordinator at the school, where 12 per cent of the 440 pupils are from ethnic minorities. “The children said the sandwiches were the sort of thing their parents might like.”

As a result, the children now have much more input into the menus, adds Mr Clark, who is also a Year 1 teacher at the school in Ashton-under-Lyne in Tameside. Where the school has introduced healthier options, it has been done in consultation with the pupils.

“When it is food they would never have come across, we felt they wouldn’t touch it otherwise,” he says. “You have to try to tempt them.”

Waste food is composted in four large bins, with the compost used on the school’s flower and vegetable gardens. But these bins only take uncooked food, salads and fruit. The rest will end up being discarded - collateral damage of a throwaway society.