The 15-year-old made the movie in her bedroom but it leaked out and now police say they are worried it could end up on a "porn site".
An officer told the Irish Sun: "She did it as a joke and did not intend it would go any further but somehow it managed to get out there.
"There is the fear that it has gone further than the smartphones and computers of pals and may be out there in the internet world possibly on a porn site somewhere.
"The girl's family is devastated and wisely the parents contacted the gardai as soon as they became aware of what had happened."
Police have now warned pupils that they could be prosecuted if they pass on the images.
One parent in Cork said: "The clip is understood to be short but very suggestive. They were specifically warned they were not to show it to anyone else.
"The children were also asked to keep in mind that there is a distraught girl and her family out there that they should be sympathetic towards."
Police are investigating how the clip got out.
What a nightmare for the family.