Another North Shore school has been hit by students' end-of-term pranks.

A sand outline in the shape of a male sexual organ was made on the cricket pitch at boys-only Rosmini College.

It is understood female students from another school were responsible.

A parent says pupilswere told at assembly the damage would cost $30,000 to repair.

But Rosmini principal Tom Gerrard says the estimate is only around $500.

He says the culprits came forward and confessed, and the incident had been dealt with.

The students concerned had offered to pay for the damage.

The parent, who has a son at the school, does not want to be named.

"The outline was huge, like you see in cornfields," she says.

A photo of it was circulating on Facebook, she says.

Westlake Girls principal Alison Gernhoefer says it was the first she'd heard of it and would be horrified if it was her students.

If it had happened during school hours, she would follow up on it, but if it was out of hours it was none of her business.

She says Mr Gerrard would have dealt with the issue.

A spokesperson for Carmel College said it was also the first time the principal Kath Deady had heard of the incident.

Police were called to Rangitoto College last Thursday to deal with two female streakers, one a current pupil and the other a past student.

Several parents have contacted the North Shore Times saying up to 37 students were suspended after being involved in end-of-term pranks. One parent says staff "tackled kids trying to take off after throwing water bombs".

Principal David Hodge says this isn't the case, although some students were sent home on early study leave after being involved in pranks.

The school was put on alert after a parent saw a former pupil buying 180 eggs in a supermarket.

A reporter visiting the school last week saw eggs strewn over footpaths and toilet paper festooning trees on the grounds.

It's not the first time Rangitoto has taken tough action over student issues.

This year's school ball was cancelled after last year's students and some parents hosted an afterball with an open bar.

Mr Hodge says he is pleased with his decision after recent incidents of other young people dying from alcohol poisoning.