Efforts to get children in England eating more healthily are stymied by poor cookery lessons, inspectors say.
Packed lunches are the most common option, survey shows
Pupils spend too little time learning how to cook nutritious meals and often end up making buns and pizzas or designing icing on cakes, Ofsted said.
In an article for the BBC News website, chief inspector Maurice Smith said lessons in food technology were not reflecting those about healthy eating.
But more than half of parents surveyed thought school meals were good.
Ofsted detected a gap between perception and reality: parents tended to think meals in schools nationally were worse than those in their children's schools.
It has produced two reports: Healthy eating in schools and Food technology in secondary schools.
The problems besetting "food technology" are traced back to the introduction of the national curriculum in 1992.
This put food in with design and technology, alongside resistant materials, systems and textiles.
At worst - "and this is more common that it should be" Ofsted said - pupils were taught "trivia".
So they might be "arranging toppings decoratively on a pizza" or using complex engineering computer-aided design software to produce simple drawings of icing on cakes.
Timetabling issues were exacerbated by larger class sizes and a shortage of specialist teachers.
The "unique" way the subject was funded - with parents supplying or paying for ingredients - was a fundamental problem.
"Teachers reported that fewer families, even affluent ones, had stocks of basic cooking ingredients, because increasingly they bought ready-prepared meals rather than cooked at home," the report said.
Parental attitudes could undermine teachers' efforts.
Ofsted quoted a girl in Year 9 (aged 13 or 14) who feared her father would say the healthy stir-fry she had devised was "rubbish" that should be "given to the dog".
In his article, Mr Smith wrote: "In recent years, pupils, parents and head teachers have increasingly argued that too little time is spent learning to cook nutritious meals and too much time is devoted to written work - the value of which is unclear.
"Our inspection findings confirm many of these concerns."
He added: "There need to be consistent messages about healthy eating across all aspects of school life, not just at meal times."
The report recommends an overhaul - noting that this has already begun under the Qualifications and Curriculum Authority.
The survey on school meals shows that only a minority of pupils take them.
The more common option - taken by 50% of primary pupils and 47% in secondaries - was packed lunches.
Meal standards were improving somewhat, Ofsted said, mainly in primary schools.
In secondary schools, pupils did not always apply their knowledge of healthy eating.
Mr Smith said: "I agree that the contents of vending machines and tuck shops need to be improved, but as long as there is a newsagent within walking distance of the school gates, sweets, crisps and fizzy drinks will always make it into the playground.
"Parental guidance and good education about nutrition will still have a major role to play."
Short lunch breaks meant many did not have time to eat meals properly or enjoy the social benefits of eating together.
Schools Minister Jacqui Smith acknowledged more needed to be done.
"That's why Wednesday's education bill contains proposals to transform the standard of school meals, after decades of neglect," she said.
But ministers also wanted to ensure children learnt about diet, nutrition, and practical cooking skills.