Could the children bring in water bottles to sip from, as they struggled with the rigours of long multiplication?

In vain I mentioned that I had been drinking water from the staffroom tap for the past 24 years and, provided there were not more than two dead pigeons in the water tank, it usually tasted pretty good.

In vain I said that in my entire 40-year career I had had less than three weeks off school, and attributed that to copious amounts of water straight from the tap.

And in vain I pointed out that the bottled stuff had probably been in the bottle for more than a year and I had recently listened to a programme on Radio 4 on which a panel of three had not been able to tell the bottled variety from the stuff piped by Thames Water.

But the staff assured me the children’s work would improve with a regular intake of “good quality” water, and since we are well on the way to becoming an accredited “healthy school”, with an abundance of fruit, good lunches and lots of exercise, I agreed to give it a shot.

The first sign of trouble came when children misinterpreted my assembly chat about the bottles they could bring. In came an amazing assortment, ranging from the tiny to the two-gallon supersize, often hauled up the stairs by doting mothers who did not want their offspring to miss out on this new school feature.

Eventually, we ruled that only a half-litre plastic see-through bottle was acceptable, and after some initial pleading that they could not identify a half-litre, children kept to this agreed size.

What they did not keep to was the rule about the contents of the bottle.

Soon, liquids of various shades began to appear at classroom doors, held by mothers with a ready excuse.

“Michelle doesn’t like water very much. It makes her throw up, you see, so I’ve diluted her strawberry-blueberry-bilberry drink a bit. I’m sure you won’t mind if she has a sip of that instead. And I’ve made it a bit fizzy for her. She won’t drink it otherwise.”

Another assembly, and another explanation about good old plain, see-through, non-coloured, uncarbonated water being the thing your body loves most. Stick to drinking that, I said, and you are bound to grow up as brainy, talented and energetic as your hunky headteacher. They laughed, and for a week or two things seemed to be going well.

Until they discovered that certain types of bottle are wicked weapons for soaking each other in the playground at lunchtimes.

Fill ‘em up from the drinking fountains, enlarge the hole in the top, and with a bit of luck and a steady hand you can “accidentally” drench your mates from 10 metres away.

There is even more fun to be had while standing at the fountain filling your bottle. Position a thumb carefully over the spout and an entire class can be sprayed as they line up. More rules were formed: bottles had to be left in classrooms at all times and were only to be used for drinking from, not as miniature water cannons.

At the next staff meeting, the water issue remained high on the agenda.

When exactly were the children allowed to reach for their bottles? Was it acceptable to leap up in the middle of personal and social health education and take a hefty swig? Should the bottles be kept on desks or at the side of the room? Should they have names on - because Ephram, having accidentally taken a quick swig from Zainab’s bottle, then clutched his neck as if he were dying and almost caused a riot?

And what were we to do about trips to the toilet, which had increased a million-fold and were seriously interrupting lessons?

After another four months, even the ardent supporters among the staff were fed up with the constant interruptions and, after the summer holiday and the fitting of new playground fountains, no more bottles were allowed in classrooms. Furthermore, we did not notice the slightest deterioration in the children’s work.

And then, just as everything had settled down again, six huge cardboard boxes were delivered to school. A free present from the Healthy Schools scheme. Four hundred plastic water bottles.

Mike Kent is headteacher of Comber Grove primary school, in Camberwell, south London