The canteen association, ASCA, which uses its buying power to deliver savings to more than 3000 Australian schools and canteens, estimates the industry is worth almost $1 billion. Association chief executive David Edwards said food companies did not make huge profits in schools but used canteens to promote their products to young audiences.

"It is an important business. Some companies do it for brand recognition. If kids buy their products when they are young then they will probably continue buying it as adults."

The association represents about 750 canteens in Victorian government, independent and Catholic schools and tries to deliver savings to schools through its buying power. All profit is donated to charity.

About three food companies approach the association every week hoping to strike a deal.

Increasing the profits of canteens and ensuring products met healthy food guidelines were the association's main aims, Mr Edwards said.

In 2013 the association severed its ties with Coca-Cola and switched to Schweppes because "they had a healthier range of products", he said.

Freedom Foods struck a deal with the association recently to supply schools with nut-free snack bars, which have been awarded a score of four out of five under a new health star rating system.

Marketing director Rebecca Carson said students were incredibly involved in their buying decisions, and she hoped the deal helped the company expand.

"Not only are our bars healthy but importantly they are also nut-free. Allergies in schools are a huge issue."

Students who bought the company's snack bars at the canteen could then ask for them to be included in their lunch boxes, she said.

An education department spokesman said clear guidelines were provided to canteen operators about what should be offered through a traffic light system, which categorises food into three categories according to their nutritional value.

"Since the policy was released in 2007, schools have been required to stop selling high-sugar food and drink such as lollies and soft drinks, and traditional canteen items like fried foods are making way for healthy lunch options with improved nutritional content. Confectionary has been banned from school canteens since 2009."

However, Obesity Policy Coalition executive manager Jane Martin is concerned the Victorian canteen regulations are not being enforced. Some food companies that made unhealthy products were reducing their serving sizes so they could be sold in canteens, she said.

"Schools play a critical role in healthy eating. Some schools are fantastic and some aren't."

Parents Victoria executive officer Gail McHardy said it was vital that contracts with food suppliers were transparent and in the best interests of children.