Headteachers say they should not be made responsible for pupils' nutrition when new regulations on school meals come into force.
A number of school leaders have expressed serious concerns to TES Cymru that unwanted new responsibilities will be forced upon them when the healthy eating in schools measure is given royal assent.
Under the legislation, local authorities and school governing bodies must take action to promote healthy eating and drinking, and governors must report on what their school is doing.
The measure is the first to be introduced by an individual Assembly member, but the regulations that stem from it will be drawn up by the Assembly government.
The regulations may specify nutritional standards as well as the maximum amounts of fat, saturated fat, salt and sugar that food and drink provided by the school is allowed to contain.
But heads said these responsibilities should be borne by specialists as they have enough work to do without having to monitor what their pupils eat.
Pat Clarke, head of St Mary's school in Overton, near Wrexham, said: "I am not trained as a nutritionist and I shouldn't be expected to be in the school kitchen checking the calorific content of school meals.
"It's not in my educational brief to do that, and it would take me away from the core teaching of children. Information should be given by properly qualified people who can come into the school."
Richard Edwards, head of Lansdowne Primary in Cardiff, already promotes healthy eating in his school with breakfast clubs and after-school cooking activities.
"We are always happy to enhance healthy eating," he said. "But we are not qualified to judge on the nutritional content of school meals.
"There's also a workload issue - what is the core function of a school?"
Anna Brychan, director of teaching union NAHT Cymru, said: "We are concerned about exactly what schools' responsibility will be. The school leader isn't, after all, qualified in these areas. We can't really support the extra burden placed on them without support and expertise.
"Schools are called upon to try to solve all of society's ills without sufficient backup."
Brian Lightman, head of St Cyres School in Vale of Glamorgan, said it was not appropriate to legislate on every aspect of children's lives.
"I think it's perfectly reasonable and laudable for schools to educate children about healthy living," he said. "But it's not our job to micro- manage the details of how the food is prepared and it's not our area of expertise."
Neil Foden, head of Ysgol Friars in Bangor, said not all schools will be prepared for the measure or even be aware of it.
"We have our meals provided by the local authority but that still requires someone at the school to monitor them and that creates an additional workload," he said.
Local authorities and schools are already required to meet minimum compulsory standards for school meals, and the Assembly government issues advice to providers on how to meet or exceed them.
The government's Appetite for Life research project has gathered information and examples of good practice around health eating in schools.
A final report into the project, being undertaken by the University of Bath is expected next summer.
A government spokesman said: "Any changes to regulations will involve consultation and engagement with all who provide school food."