But my next request - for 20 pickled onion Space Invaders - relieved their fears. The order was, of course, for the tuck shop.

Tuck, nosh, grub - these are all words I associate with the world of Greyfriars and Mallory Towers. A world of super suppers and midnight feasts, where kind aunts sent tuckboxes full of glorious goodies for all the dorm.

So when I found myself supervising the sixth-form tuckshop, I assumed the buns and the bull’s eyes would long since have been left behind.

These health-conscious young ladies would surely be nibbling peanuts or munching on apples, maybe indulging in a raisin or two.

But I was mistaken. It was not just Karate Belts they were after, but Beer Bottles, Giant Strawbs and Ring Pops too - candy dummies in garish colours.

So have all our admonitions about a healthy diet been in vain? Or do the stresses and strains of sixth-form life drive them towards the nearest Lion bar?

Perhaps those dummies hold the clue and these girls are simply retreating to the safe world of the juniors, where neither figures nor A-levels matter at all.

From one end of the school to the other, it would seem that junk food has become a way of life.

These children are the new breed of grazers. If they are high on chocolate by the afternoon, they are often pale and listless when the day begins.

They come to school ill-prepared for a long day’s work. Many of the sixth form claim they never have breakfast and even among my class of Year 7, a third often arrive on empty stomachs.

How can we expect them to concentrate? Lunch is late in a secondary school and so they snack their way through the long morning.

For some, tuck is a mainstay, not an extra - and no longer comes in the shape of a bun or even that regulation bottle of milk.

Along with this grazing has come the steady decline of the family meal. We all know that sixth-formers, as they dash between their jobs and their social life, have no time for such old-fashioned customs, pausing only to grab a handful of Karate Belts on their way out.

But the juniors? Their lives seem frantic, too - swimming, gym club, music lessons, ballet - and meals, like homework, get slotted in wherever there is a space. A fifth of my Year 7 only eat as a family once or twice a week. And if they do sit down together, the chances are the television will be on.

Perhaps I should subvert the sixth-form order. Perhaps I should ask for buns and sandwiches - good, old-fashioned boarding school tuck. They wouldn’t sell, of course, but it might ease my conscience.

For if I order the Pot Noodles and Choc Dips, am I not condoning this way of life? You can make healthy profits from unhealthy habits. If selling Karate Belts has become a moral issue, then I am fast becoming a pacifist.

Margaret Leeson teaches in Kent.