Parents are defying the school 'packed lunch police' and giving their children banned sweets and snacks.
But some children have been caught red-handed and their teachers have confiscated 'naughty' food.
More than one in four parents who send children to school with a packed lunch admitted they had broken guidelines over the contents.
Crisps, chocolate and biscuits were the most popular illicit foods.
Crisps had been slipped into lunchboxes by 54 per cent of parents and 47 per cent had packed frowned-upon chocolate bars.
A survey of 1,189 parents was carried out by YouGov for The Grocer and Bacofoil. More than a third said schools should not control what children eat, although 38 per cent said they packed fruit.
The Grocer said: "Our kids are being watched. Schools are set strict guidelines and at times they are enforced with an iron fist."
Adrian Brown, MD of Bacofoil owner Wrap Film Systems, told The Grocer: "Parents are more conscious about what they put in their kids' lunch boxes and in some ways more imaginative than ever.
"Our product are being used for a variety of uses, such as wrapping vegetable sticks."
Earlier this year, a head teacher defended his decision to permanently exclude a six-year-old pupil after the boy's parents claimed he was punished for having Mini Cheddars in his lunchbox.
Riley Pearson was kicked out of Colnbrook CofE Primary School in Slough, Berkshire, for breaching his school's healthy eating policy.
Headteacher Jeremy Meek said at the time: "In line with many other schools, we have banned items such as crisps, chocolate, sweets, cakes, fizzy drinks and biscuits.
"Where there is a persistent and deliberate breach of school policy, such as bringing in crisps, biscuits, sausage rolls, mini sausages, scotch eggs and similar, and all other avenues have been exhausted, the Governors would expect further action to be taken.
"If we are faced with a situation where a parent threatens to send a pupil into school with insufficient food to sustain them throughout the school day, it is a risk we simply cannot afford to happen."