The SCC said pupils are consumers who make their own decisions
More needs to be done to encourage children not to choose chips, pizzas or sweets for lunch, it has been warned.
The Scottish Consumer Council has raised concern over pupils shunning school meals for high street fast food.
It called on the Scottish Government and retailers to play their part in ensuring pupils ate well if they went out of school for lunch.
The number of secondary school pupils choosing to eat school meals has fallen from 44.9% in 2007 to 42.9% in 2008.
SCC Chair Douglas Sinclair said: "While the core to improving lunchtime choices for Scotland's teenagers is about supporting them with the information about good food and healthy eating, the Scottish Government and local councils have to play their part as well.
"These young people are consumers who are making their own decisions.
"They will only stay in school if what's on offer there in terms of both the food and the lunchtime environment is better than what's on offer outside."
An SCC spokesman added: "The Out to Lunch report suggests the obesity time bomb has still not been defused for many young Scots, but it is not something that schools - or parents - can do on their own."
The Scottish Government said it agreed that schools alone could not change the "eating habits of future generations" and that was why it was taking action on a number of fronts, including a recently-launched Obesity Action Plan.
A spokeswoman said: "Beyond the school gate, we acknowledge the challenge posed by chip vans.
"However, some local authorities - East Renfrewshire and West Dunbartonshire and more recently Renfrewshire and Falkirk Council - have taken steps to prevent them trading near schools at lunchtime.
"Others have introduced their own vans with healthy options. We would encourage others to make best use of the current rules and regulations and look at best practice across the country to address this issue."