Screenshot from a video shows the girl being slapped by another student. [Photo/weibo.com]

Five middle school students who slapped a girl for not wearing proper school uniform are now under police investigation, according to a report by Beijing News.

The incident caught the attention of Chinese netizens after a video of the assault was uploaded online on Monday.

In the minute-long footage, the victim didn't fight back although she was repeatedly slapped by several students in turn for more than 10 times. One of the attackers appears to be smoking.

Later reports said the bullying happened in Jiangshan Second Middle School in Jiangshan, Zhejiang province.

The five attackers, all in the ninth grade, saw the eighth grade girl not wearing her school trousers on December 12. Two days later, the five students found the girl and took her to the attic of the school building. Four students struck the girl while the other recorded the incident.

Zheng Chaolong, the school's deputy principal, said the school pulled up the five girls and told them to apologize to the victim on Dec 15, 2014.

Zheng admitted that the incident has exposed problems in the school's management and promised to provide further psychological consultation to the victim.

Local police said the public will be informed of the investigation but care will be taken that the children involved are not unduly affected.

School bullying has been on the rise in China in recent years. A 2012 study by researchers from Sun Yat-sen University in Guangdong province found that of the total 8,342 middle school students who were surveyed in four cities in the Guangdong Province, 20.83 percent were involved in bullying, 18.99 percent were victims of bullying, 8.60 percent were bullies themselves, and 6.74 percent were both bullied and bullied others.

In a recent case, a college girl in Shunyi District of Beijing was beaten by four fellow students, resulting in her psychological damage. The attackers were later sentenced to six month in prison.