Retailers will be asked to steer pupils away from deep-fried foods and high-sugar drinks, particularly when offering cut-price "meal deals".
Instead, they have been asked to promote sandwiches, soups and salads alongside unsweetened fruit juice or reduced-fat milk, and are being given tips on how to increase the sale of healthier foods in their stores.
But Green MSP Alison Johnstone called for action on the issue to go much further than the new Government-backed plans.
She said: "Given the dominance of supermarkets it is disappointing that what has been published only invites retailers to consider what they might do.
"I want to see local communities, schools and food outlets given real support to improve the health of young people and that requires funding and political will rather than voluntary schemes that can be ignored."
The Government initiative to discourage pupils from eating junk food on their lunch break is known as the Beyond the School Gate scheme.
It aims to promote healthy eating for children when they are outside school by encouraging shopkeepers to think about the food they are selling to youngsters.
The project is also promoting ways to encourage children to stay on school premises at lunchtime to eat a healthy meal.
Suggested ways of achieving this goal include offering better school cafes with more appealing food and drink.
Research carried out for the project has found that more than half of secondary school pupils buy lunch outside school at least once a week.
Of a total of 938 young people who responded, more than 70% had at least four food outlets in the immediate vicinity of their school.
The most common shops visited were large supermarkets, small convenience stores and bakeries.
After smoking, bad eating habits are the second major cause of poor health in Scotland, with research showing the national diet contributes to a range of serious illnesses including coronary heart disease, cancers, strokes, osteoporosis and diabetes.
Public health minister Michael Matheson was in Glasgow to launchd the Beyond the School Gates scheme.
He said: "We have made excellent progress in improving the standard and nutrition of school meals in Scotland, but the evidence shows many children are simply not staying to eat these meals.
"We are fighting a battle against obesity in Scotland and we know that if children eat unhealthily in their teenage years it's a habit that can be hard to break.
"That is why this is a battle we want to take beyond the school gates.
"This initiative looks at how we can encourage children to choose healthier options both inside and outside school."
The project is also backed by the Scottish Grocers' Federation.
John Drummond from the federation said: "It is having a real impact in our communities and shows the highly active role that retailers play in offering healthy eating choices and promoting healthy foods, particularly fresh fruit and vegetables."
Peter Johnston is health and wellbeing spokesman for the local authority umbrella body, Cosla, which backs the scheme.
He said: "Councils have done much to improve the food on offer within our schools in recent years and are working hard to encourage more pupils to take advantage of this more often."