"A whole load of nasty things goes on outside schools and you've got to be aware of what goes on," Joe Leiper, head of Oldmachar Academy in Aberdeen, and a member of the national inquiry team on school meals, told the conference.
Burger and chip vans offered rival services to secondary pupils and pulled them away from the more secure school environment. Smoking and drugs were other lures outside, he said.
Vans and shops outside schools also sold precisely the junk food that was now being strongly opposed by the health-promoting school movement.
Mr Leiper said he spoke to one boy at a west of Scotland secondary he visited last year as part of the inquiry team that produced the report, Hungry for Success. The boy had Irn Bru and deep-fried pizza five days a week for lunch, outside school. His friends ate Pot Noodles or chips with gravy.
At Oldmachar, it was a similar story until 1997 when the concept of the health-promoting school was introduced. Chips were served at the breakfast club, at morning intervals and at lunchtime, where it was "chips and cheese, chips and gravy and chips with everything". Catering vans outside the school were also competing for business.
"We found some resistance to change and some of that wasn't just from young people," Mr Leiper said. The headteacher, who retires next week, said pupils were consulted on what they wanted, which included pre-ordering healthy options in the morning to avoid lunch queues. The school also staggered lunchtimes for the 1,120 pupils to give them time to eat, and played music to lift the experience. "We do not chase them out. Some schools get the disinfectant out on the tables after 20 minutes to get them out. No, get them in," he said.
Young people had to buy in to strategies, especially in secondary, said Mr Leiper. Primaries were more fertile ground for promoting healthy eating.
Headteachers had to place diet and health on the development plan if there was to be any chance of success. They had to lead from the front.