The police force said the images are not indecent, but they were on an pornographic website.
The BBC reported that 731 photographs of Northern Irish schoolgirls appeared on a website used by paedophiles.
It said the schoolgirls included pupils from 19 secondary schools in Northern Ireland.
The BBC said the photographs were taken or uploaded to social media by the girls themselves and then taken from those accounts without their consent.
PSNI Detective Chief Superintendent George Clarke said police were made aware of the images on 10 October.
He said: “Following consultation with the PPS, which has confirmed that the images do not appear to meet the criminal threshold for indecency, we have agreed with them to continue to investigate the matter to ascertain if any other criminal offence can be identified.”
Chief Supt Clarke said the website is based in the Netherlands and is therefore outside of the PSNI’s jurisdiction.
He said: “International Letters of Request have to be prepared in order to secure the information by way of assistance from the Dutch Authorities.
“Since there is no criminal offence yet identified that meets the threshold for the ILOR, this information is not compulsory for the website to provide.”
Chief Supt Clarke said the PSNI made contact with the website in question.
He said it removed the images, but it would not say who had uploaded them to the site.
“It is important to note that the website is under no legal obligation to provide this information or remove the photographs. The images have been removed,” he said.
“We have made contact with six schools and provided internet security advice and reassurance where needed.
“The fact that no crime has yet been identified was also highlighted.”