A principal has been forced to take drastic measures to stop vaping at her school - with a photo shared on social media of her apparently peering under a toilet cubicle.

Green Bay High School principal Fiona Barker told parents the picture - which has been circulating on social media - showed her trying to work out how many pairs of feet were visible in the toilet after she was alerted that two girls may be vaping inside.

It comes as some Auckland schools have been resorting to locking toilet blocks to deter students trying to have a sneaky vape.

"Like many things on social media, photos can often provide a one-sided version of what is happening at the time," Barker wrote in an email on Thursday to whānau of students at the west Auckland school.

"We take our duties very seriously around monitoring what is happening at our school in order to be assured it is a healthy and safe environment," she wrote. Vaping had become an issue in schools across the country.

"I was responding to concerns that there were two students in the same cubicle potentially vaping. The photo shows me checking from a distance the number of feet in the cubicle.

"As vaping products are R18 only, potentially highly addictive, and prohibited at schools, it is important that we respond to these situations promptly and strongly.

"It is also important to note that we would never jeopardise someone's privacy and dignity in the toilets."

It comes as some schools have resorted to locking toilet blocks in an effort to curb the trend. Principals say vaping is more socially acceptable than smoking, as well as being easy to hide and vaping gear being easy to buy. There is also less research into the risks.

The number of Kiwi teens trying vaping has skyrocketed in recent years, despite new legislation making e-cigarettes R18 for purchase.

Almost 40 per cent of 15- to 17-year-olds had tried an e-cigarette at least once in the Ministry of Health's 2019/20 NZ Health Survey - up from 25 per cent a year earlier.

The number of those who used an e-cigarette at least monthly more than doubled, from 3.4 per cent to 8.7 per cent.

However "current smokers" in the age group fell from 3.8 per cent to 3.3 per cent.