A group of secondary school boys have been assaulting young women in broad daylight and posting videos to social media.

New Zealand Secondary Principals' Association executive member Patrick Walsh - chairman of a high-powered Government group to counter cyber bullying among school students - said the boys would film themselves slapping young women on the bottom.

The boys were caught when a shopper who witnessed them in action, recognised their uniforms and reported it to the school.

They were identified and confessed to school bosses.

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"Teen boys would jump out of a car and approach young women and smack them on the bottom," Walsh said.

"That was recorded and they'd jump back into the car and drive off."

He declined to name the school but said the principal was dealing with the crude antics.