By Justin Parkinson
BBC News education reporter at the NAHT conference
Healthy but boring school dinners could be encouraging children to buy their lunch from chip shops instead, a head teachers' leader has warned.
Jamie Oliver promoted healthy school dinners in a TV documentary
Mick Brookes, general secretary of the National Association of Head Teachers, criticised a regime of "gastronomic Puritanism" at its annual conference.
Such measures will not help overcome childhood obesity, he said.
Schools need "greater freedom" to devise menus for pupils, Mr Brookes told delegates in Harrogate.
Following TV chef Jamie Oliver's campaign for healthier school dinners, the government promised £280m to raise standards in England, including more stringent nutritional guidelines.
The number of obese or overweight children in Britain aged between two and 15 has risen to about 30% in recent years. Meanwhile, poor diet accounts for around a third of deaths from cancer and heart disease.
Mr Brookes told delegates: "While we agree that childhood obesity does need to be challenged, we will not achieve that aim by creating a kind of gastronomic Puritanism that will simply encourage our young people to leave the premises at lunch times to go to the local chip shop, or to bring lunch boxes full of contraband."
In his end-of-conference speech, he also warned of a "parental backlash" against unqualified classroom assistants being left in sole charge of some lessons.
Mr Brookes also described England's school inspections system as "probably the most aggressive and punitive in the developed world".
Schools were dissuaded from admitting problems, he said, because of a fear of "public pillorying".
He said: "So why not ask Ofsted teams to highlight areas for improvement; authorise a package of support and give the school a year to improve performance in partnership with colleagues. At the end of that time that aspect of the school's performance could then be re-assessed by HMI."