Schools need help in trying to combat a rise in "happy slapping" attacks, a teaching union has said.
Both attacks appeared on the YouTube website
The warning follows two incidents where people in Cardiff, including a chip shop worker and a 15-year-old girl, were filmed being punched and kicked.
Both so-called "happy slapping" clips were posted on a video website.
The National Association of Head Teachers (NAHT) said parents, police and the government needed to help schools deal with the problem.
Dr Chris Howard, from the NAH in Wales, said teachers had to have more assistance to fight the trend of posting attacks on to sites.
"Some of that material involves bulling and violence - it should not be there," he added.
"[The children] are creating online communities - which are, if you like, secret gardens defying adults to get involved in their discourse online."
One of the films was 23 seconds long and apparently showed the chip shop worker being attacked by a schoolgirl aged about 15 as cheering children looked on.
The incident took place in the Tremorfa area of Cardiff.
The clip of the chip shop incident was viewed around 4,000 times before it was removed from the video website YouTube on Friday.
The footage begins with the schoolgirl fighting with the chip shop worker before both of them fall to the ground.
The schoolgirl, who appears to be wearing school uniform from the local Willows High School, leaps up and attempts to kick and punch the woman, before the worker walks back into the shop as watching children cheer.
YouTube, which enables web users to post their own videos, has become one of the internet's most popular sites.
Meanwhile, a girl, 15, has spoken of her "disgust" after footage of her being attacked was also put on the web.
The teenager was left with bruising to her head and body after her female attacker, sitting astride her, delivered a series of blows.
"I feel upset because everyone is going to know that I am a wimp," she told the BBC's Radio Wales.