By Sean Coughlan
BBC News education reporter
A school which banned unhealthy food and drinks from its vending machines has had to change back to a more teenager-friendly range of snacks.
School vending machines can raise £15,000 per year for schools
Queensbury Upper School in Dunstable installed machines with organic crisps and healthy options - but pupils "voted with their feet" and did not buy them.
Instead the school has moved to a compromise, with machines without fizzy drinks, but with brand-name snacks.
Education Secretary Ruth Kelly is calling for healthier school food.
In a speech to the Labour Party conference, Ms Kelly announced a ban on junk food in school meals and said that from next September, school vending machines will not be allowed to sell chocolates, crisps or sugary fizzy drinks.
But the experience of Queensbury Upper School suggests that persuading young people about the benefits of healthy food might not be as straightforward as persuading their parents.
The Bedfordshire school made news headlines by announcing it was going to change to a healthier range of food in its vending machines - including rice cakes, vegetarian snacks, fair trade chocolate and bottled water.
The secondary school had 14 vending machines - and in particular there were concerns among staff that fizzy drinks were adversely affecting some pupils' behaviour.
"There was a need for change. Staff had noticed a deterioration in behaviour - and even though it couldn't be put down only to the drinks, there was a feeling that there was a link," said assistant head Karen Hayward.
And removing the fizzy drinks had been a success in improving behaviour, she said, with a "radical reduction" in the incidence of "hyper" behaviour among pupils.
But the new range of machines, without the familiar snacks, were not a big success with the pupils - and after persevering for 18 months, the school gave up the experiment with the organic vending machines.
Instead, it has now moved to what she says is proving a successful compromise.
There are four machines which sell a mixture of brand-name chocolates and snacks - but not high-sugar fizzy drinks. Instead there are water, milk and juice drinks - and the products are intended to be at the healthier end of the snack range.
"Behaviour has changed for the better from changing the drinks," she said.
And the school was trying to encourage more healthy eating through its canteen - with salads proving popular.
But she warned against under-estimating the consumer instincts of teenagers - and how their brand awareness influences their choice of snacks.
She said the change from the original range of fizzy drinks and snacks has lost the school about £4,000 to £5,000 per year. But she believed the financial loss was outweighed by the improvement in behaviour.
A report last year from the Food Standards Agency, which argued that healthy vending machines in schools could be profitable, said secondary schools had earnings of between £10,000 and £15,000 per year from machines.
There are also other steps to be taken in creating a more healthy lifestyle for children, Ms Hayward said.
This could be something as basic as encouraging pupils to drink water during the day.
"Diet and learning are linked," she said.
From next year, teenagers in the school will also have to do without the chocolate snacks in the vending machines - and they might have to return to more healthy products.
A company promoting healthy school vending machines, called H-Boxes, with snacks that are low in fat, sugar and salt, says that it has machines in 64 schools across the UK.
The push for healthier vending machines has also been taken up in the United States - with school systems in a number of states imposing restrictions on fizzy drink sales in schools.