Mt Maunganui school students have been targeted by "sexually explicit" posts on social media.
Police are now involved and parents have been asked to educate their children about internet safety.
Mount Maunganui Intermediate School told parents that the sexually explicit Instagram posts made untrue allegations about "a wide range of students".
More than 120 people, including students, may have seen the material.
The school has instructed students about slander, bullying and trolling after learning of the posts when teachers and students supplied staff with screenshots of the material.
The posts were made over the weekend.
School principal Lisa Morresey said an unknown number of male and female students were identified in the posts, which were made on personal devices outside of school hours.
She said students had been told at an assembly that allegations made in the posts were untrue. They had also been informed about cyber safety legislation, slander and the spreading of false information.
They were told that they should not be on social media until they were 13 years old, and instructed that they should not follow social media feeds unless they knew the person posting them.
"In the old days, people would write on bathroom walls," Morresey said. "Now, in the world of social media, there's another platform for that."
Instagram allows users to share photos and videos publicly and privately. However, the Mt Maunganui posts were based on text, rather than images.
The school had advised police about the material but the officer dealing with the case could not be contacted yesterday.
Morresey said that no offender had been identified.
In the old days, people would write on bathroom walls. Now, in the world of social media, there's another platform for that.
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The school this week urged parents to educate children about internet safety, suggesting that they use a system based on the acronym Star. This stands for "Screenshot", "Tell an Adult", and "Report".
Morresey said social media issues had become common in schools. Staff "regularly deal with friendship and relationship issues that stem from students' social media use".
The president of the Western Bay of Plenty Principals' Association, Dane Robertson, said that he was unaware of any wider problem of sexually explicit social media posts affecting schools in the region.
"We advise our students that, if they come across anything like that, they should seek adult help. Then it becomes a police matter."
Netsafe, an independent body aiming to make New Zealanders safe online, advises people targeted by bullying or explicit posts to collect evidence of the behaviour, such as by taking screenshots and recording web addresses.
Under the Harmful Digital Communications Act, offenders who "cause harm" with social media posts can be jailed for up to two years or fined up to $50,000.
What to do if you are targeted by cyber bullies (Star):
Tell an Adult
Where to get help:
Lifeline: 0800 543 354 (available 24/7).
Suicide Crisis Helpline: 0508 828 865 (0508 TAUTOKO) (available 24/7).
Youthline: 0800 376 633.
Kidsline: 0800 543 754 (available 24/7).
Whatsup: 0800 942 8787 (1pm to 11pm).
Depression helpline: 0800 111 757 (available 24/7).
Rainbow Youth: (09) 376 4155.
Samaritans: 0800 726 666.
If it is an emergency and you feel like you or someone else is at risk, call 111.