A controversial new edict has barred secondary school pupils from taking a sip out of their water bottle during lessons, with the college saying this causes "over-hydration" and an exodus to the toilet.

Whanganui High School Principal Ross Brown cited "classroom disruption" as the reason behind the ban and said there would be consequences for rebel hydrators, but other schools spoken to had no problem with classroom swigging.

"We've had issues [with] kids over-hydrating then needing to leave to toilet, kids wanting to get their drink bottles filled up...just classroom disruption.

"When there's kids saying, 'I have to go to the toilet', you don't want to call their bluff on toileting."

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Instead of being able to drink water in class, he said pupils now waited until the breaks between periods.

"Every 55 minutes they get a chance to have a drink."

Brown said a class at the school wrote a letter of complaint to him about the ban, but once he went and discussed it with them they understood.

"They quoted a lot of things about teenage brains [and] how they had to be hydrated, and they do."

Brown was also encouraging pupils not to be excessive with their hydrating.

"I had a boy with a two-litre bottle and he was walking around [with it].

"We had a chat about it and he left it here and I took him a drink and he put that in his bag."

Water in class was also getting spilled on technology or in science labs, "so it's not something that's unreasonable at all I think".

He said if pupils broke the rules and tried to drink water in class there would be consequences.

"When you're running a school you want to have something that the masses abide by and understand."

Water was still a healthy part of the Whanganui High School diet, Brown said.

Dannevirke High School principal Dr Dawid de Villiers said the school did not only allow drink bottles, but actively encouraged it.

"We have quite a few water fountains with drink bottle attachments on the grounds.

"Students can take the bottles wherever they go.

"Our approach is to educate rather than restrict, as students spend the majority of their time outside of school, where they have to make healthy choices themselves."

Studio Rubix dietitian Kate Morland said changing the way water was consumed might not change toilet habits for pupils.

"If a student is hydrating at breakfast, morning tea, lunch, afternoon tea and then after school that should be adequate...[but] I don't know if it would make a huge amount of difference [about] going to the toilet.

"Some people have quite weak bladders.

"I think it's a hard one to try and police. It's healthy habits – we have to go to the toilet regularly throughout the day."

Morland said good practice would be for teenagers and children to drink a decent amount of water every two to three hours.

"Sipping bottles see kids having a sip here and a sip here and [they] never actually hydrate.

"I can actually see a bit of a positive benefit around it."

Central Normal School principal Shona Oliver said their children could use drink bottles at any time.

"We have places for children to fill their water bottles."

Palmerston North nutritionist Gaye Philpott said between six to eight drinks of water a day was recommended.

"That might look like a drink at meal time and a drink between meals."

Philpott said if pupils were drinking less water, it might mean they have difficulty concentrating and they might suffer headaches.

Carncot School principal Chris Michalski said children could bring their own bottles to class and use them in class, but they were put aside on a bench.

"They then have access to them for breaks, or for a sip every hour if needed.

"Having them aside on a bench prevents this constant sipping at the desk, which is not necessary."