A high school student's leg was "cut to the bone" when he was struck by a schoolgirl in underwear riding a dirt bike in an end of year prank that went horribly wrong.

It is understood video footage of the accident is now circulating and that the six Waikato Diocesan School schoolgirls involved dropped a GoPro camera at the scene.

The victim, a Year 12 boy from Hamilton Boys' High School, suffered a deep gash to the leg when four Dio girls drove on to the boys' field at lunchtime yesterday in what has been described as an annual streaking prank.

A source told the Herald the victim was engrossed in a game of football and did not notice the approaching bikes.

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One of the bikes struck the boy, causing the girls to fall off. One of the girls is alleged to have given the middle finger before driving off.

The victim lay on the field in agony until he was eventually stretchered off the field and taken to hospital. He is now recovering from his injuries.

"Apparently they weren't wearing anything apart from the helmets," a source said.

Posts on social media refer to the boy's wound as a "very deep cut", possibly caused by the motorbike pedal hitting his leg.

"Doctors said it's actually down to the bone."

Another source told the Herald the annual streaking prank had been occurring since at least 2009.

A student told the Herald teachers had been aware of the prank for the past three years and stopped it.

"However this year they used motorbikes."

Other reports say the girls were wearing underwear, shoes and helmets and going a "decent speed".

Police told the Herald the victim suffered moderate injuries and was taken to hospital.

Waikato Police Senior Sergeant Ray Malcolmson said the prank happened just after midday and police rushed to the school after receiving a call from ambulance staff.

It is understood the boy's family are speaking to police today.

"There was a collision with a bike and a student on the school grounds. The student has suffered moderate injuries and has gone to hospital," police said.

Waikato Diocesan principal Mary Curran confirmed six senior students were involved in the "unfortunate" prank.

Curran said school authorities were helping police.

"This is unacceptable behaviour, which we are taking very seriously," she said.

"Due school process will be followed."

Curran said she wished the injured student a speedy recovery.

Hamilton Boys' High School principal Susan Hassall told the Herald: "I am unable to make any comment at this stage, as I have been away from the school at a meeting through the afternoon and early evening."

She had earlier confirmed "uninvited" students from another school entered the grounds.