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Oranga School is one of the first in the country to become a "water-only" school and no one is exempt from the rule - not even the principal.

The One Tree Hill school is so intent on improving the health of its students that principal Juliet Small had to hang her head in shame when caught with a bottle of energy drink.

Ms Small said parents had been hugely supportive and even the children were happy - except when they saw her carrying a sports drink.

"I was warned by one of the kids, so I've not made that mistake again."

Many primary schools have banned sugary fizzy drinks and some actively encourage a water-only philosophy. But at Oranga, there is a blanket ban on everything except water.

"There are all these energy and juice drinks now which have just as much sugar as cola, so where do you draw the line?" said Ms Small.

"We decided that water was the only safe bet."

Classrooms have signs declaring "water-only area" and a daily amnesty at the school gates allows rogue drinks to be handed in and returned at the end of the day.

Juice and flavoured milk drinks are off the lunch menu and children are asked to bring their own bottles to be filled with water each day.

Alistar Duncan, 8, said he felt more awake at school now he was drinking more water. He had even started choosing water at home instead of juice - "most of the time".

Classmate Battrina Mairuria, 9, admitted she missed fizzy drinks sometimes, but liked the change because "water is healthier".

The water-only policy was adopted this year after suggestions from staff.

Ms Small said it was particularly welcomed by the dental hygienist because the school had fluoridated water, which many of the children did not have access to at home.

The water was having other benefits as well. Visitors had commented on how calm the environment was and one student had said her classmates "did not get so high".

Sugary drinks are blamed for contributing to the obesity epidemic in New Zealand, as well as being a factor in causing diabetes.

Dr Robyn Toomath, spokeswoman for the Fight the Obesity Epidemic, said she had not heard of any other schools taking such a line.

"It's absolutely what's required and top marks to Oranga."

But Ms Small admitted there was one exception to the rule. Coffee was still permitted in the staffroom.