In a 99-second video clip circulating online, a teenage girl was slapped on the face by another girl more than 30 times. The attacker also kicked the girl's stomach and pulled her hair.
It was the latest incident in a round of violence both perpetrated by and targeting teenagers in China.
Police in Nanchang City in east China's Jiangxi Province launched an investigation after the victim's father reported the case to police last week.
Police said on Friday that a preliminary investigation showed that two teenage girls carried out the violence, recorded the attacks, and put the video online. The attacker has been detained, while the videographer is still on the run.
The 13-year-old victim, Xiaowen (an alias), was discharged from the hospital on Thursday, where she was treated for soft tissue injuries.
The student at Xianghu Experimental Middle School said the beating occurred after she was asked to leave the school by a girl surnamed Hu, who she had studied with in the school two years ago.
Hu took her to a rooftop platform on a 16-story building near the school, where the other girl beat her while a dozen other teenagers watched and Hu recorded video of the scene.
Xiaowen said she thought Hu planned the violence, and that the beating may have been motivated by a dispute she had with Hu when chatting online not long ago.
Chinese parents were appalled by the video and worried about the safety of their teenagers at school.
Xiaowen's father said he could not understand how children could be so brutal to their peers over small disputes.
Last month, the Supreme People's Court reported 67 cases of teen violence this year. The average age of the abusers was between 15 and 17, and the victims were aged 14 to 16.
The cases involve fighting, robbery and sexual assault. In some, abusers recorded the scene and put video online, often causing repeat psychological damage to the victim.
The Supreme People's Court said the cases suggest the students involved were younger than previous years and girls were more likely to be involved. The teenage offenders were also more inclined to show off their violence online, according to the court.
Zhou Xiaozheng, a social science professor with Renmin University of China, said the cases suggest that teenagers are likely to be influenced by violent behavior from their parents and the availability of violent scenes online.
