Efforts to promote healthy eating among secondary pupils are being undermined by short lunch breaks and food technology lessons which allow too little time for cooking, Ofsted said this week.
The fast-food culture of many schools gives catering staff too little time to influence meal choices as well as depriving pupils of the social benefits of eating together.
In food technology lessons, children spend too much time on theory and too little on practical work, inspectors said.
Teaching is hindered by short lessons, large group sizes and the need to ensure poor families can afford the ingredients.
The findings are published in two Ofsted reports, Healthy eating in schools and Food technology in secondary schools.
Despite publicity about the poor quality of meals, catering had improved in only a minority of schools, with primaries making greater progress than secondaries.
A poll of 401 parents published alongside the reports found only a third thought the quality of meals at their child's school was good or better.
This increased to more than half when only parents whose children have school dinners were questioned.
Primary parents rated meals offered by their children's schools more positively, with only one in 12 saying they were poor compared to one in nine secondary parents. But parents of primary children were more likely to take a negative view of school meals nationally. More than a quarter said they were poor, compared to 17 per cent of secondary parents.
Overall, three-quarters of parents said healthy options were available at their child's school, but most secondary parents said their child would not choose them.
Healthy eating in schools found teaching about nutrition is good or better in both primaries and secondaries, but secondary pupils often fail to use that knowledge to make healthy lunch choices.
Vending machines and tuck shops in secondaries often sold items that undermined healthy-eating messages. Staff also often lacked understanding of key concepts such as "healthier foods" and "nutritional standards", it said.
The report, based on visits to 10 schools and inspection evidence from 50, said all schools should have a food policy, and called for training for catering staff. It also called for parents to be consulted on schools' decisions about food.
Food technology in secondary schools supported concerns, expressed by pupils, parents and schools, that too little time is spent learning to cook nutritious meals.
Lessons showed "a lack of clarity about the relationship between the teaching of food as a life skill and the use of food as a medium for teaching design and technology," it said.
The report, based on a survey of 30 schools, said: "Pupils are required to engage in complex product development before they have an adequate understanding of food ingredients, nutrition, hygiene and cooking skills."
Maurice Smith, chief inspector, said that all aspects of food technology in the curriculum needed to be scrutinised, including the quality of teaching and training.
John Dunford, Association of School and College Leaders general secretary, said schools chose short lunch breaks because they reduced disciplinary problems.
Louise Davies, deputy chief executive of the Design and Technology Association, said: "Even though food technology delivers a vital life-long set of skills and knowledge, the focus on literacy, numeracy and ICT in our schools has meant that lesson times have been cut, class sizes have grown and ingredients are under-resourced for effective teaching. In some schools departments have closed because they have been unable to recruit staff."
Average school spending on ingredients was 24p per pupil per lesson, she said.