This newspaper understands the video features the underage girl, who is easily identifiable, performing a sex act on a teenage boy of the same age.
It was reportedly filmed by a second girl, who attends a school elsewhere in West Yorkshire, before being widely shared via social media.
Police said yesterday that there was no active criminal investigation in relation to the video, but it prompted one secondary school in Leeds to draft in specialist officers.
Year Nine pupils were given a hard-hitting talk by police about the dangers of sharing sexual or naked images or videos of themselves or others – commonly known as ‘sexting’.
In July, it emerged that more than 4,000 children in England and Wales have been dealt with by police for sexting since 2013.
Superintendent Matt Davison, who heads up neighbourhood policing teams in Leeds District, said: “The police are regularly made aware of concerns raised by schools and parents about pupils being involved in sexting or the recording and sharing of images of sexual activity.”
He said safeguarding young people was a key priority and criminal investigations would be conducted where adult involvement was suspected.
Where all parties involved are underage, the focus is on advice, education and any welfare issues.
One parent, whose child attends the secondary school in Leeds, said: “There has been speculation in the school for the last 10 days or so over a video going round of a girl.”
She was shocked though on Thursday when her child described an assembly, during which a number of police officers were present and a “heavy handed” warning was issued.
It had left her child believing they would be questioned under caution or have their phone seized because of the video.
The assembly was followed by an email to parents which suggested checking their child’s phone and deleting content.
Supt Davison said: “A Safer Schools officer has visited a Leeds secondary school this week to give a talk to pupils following a request from the school.
“There is no active criminal investigation in relation to the concerns raised there.”
He said the Safer Schools team would continue to work with partner agencies to offer support to all schools in the city.
The school in question declined to comment.