An Adelaide high school student has been left requiring surgery after having her front teeth knocked out during an alleged group assault, which is currently being investigated by police.

Key points: The 13-year-old victim is in hospital, and was on her way home from school when she was attacked

The 13-year-old victim is in hospital, and was on her way home from school when she was attacked Police have charged two girls aged 14 and 15 with aggravated assault

Police have charged two girls aged 14 and 15 with aggravated assault A family friend says the alleged assault was filmed and uploaded to social media

WARNING: This story contains an image that some people may find distressing.

Police have arrested two girls — aged 14 and 15 — over a fight between a "group of teenagers" at McDonald's in Gilles Plains, in Adelaide's north-east, about 3:45pm on Tuesday.

Both girls have been charged with aggravated assault causing harm.

"A 13-year-old girl was assaulted and sustained facial injuries," police said in a statement.

"She is currently in the Women's and Children's Hospital awaiting surgery — her injuries are not life-threatening."

The arrested girls will appear in the Adelaide Youth Court at a later date.

In a social media post, a friend of the victim's family said the young girl — who the ABC has chosen not to identify — had been "left absolutely damaged physically and mentally".

The woman said the girl was on her way home from school when she was attacked, and that footage of the incident had been shared online.

"[She] just started high school a week ago. She is a beautiful straight-A student," she posted.

"She has suffered bullying all her life."

The victim is awaiting surgery for her injuries. ( Supplied: Victim's family )

The woman said the teenager was the victim of a "cowardly" attack and was "punched her from behind without her even knowing".

"She has lost her two front teeth, suffered major memory loss, concussion, neck, spine and shoulder damage," the woman wrote.

"She is currently still in hospital having numerous CT scans, X-rays and surgery … her life is changed forever.

"To make matters worse, the culprits and kids nearby videoed it all and it is now being sent around via social media."

The friend said the girl would need numerous surgeries and dental treatment and had suffered "permanent damage to her face".

She said the girl's mother was "absolutely devastated".

In its statement, police said it "was not a random incident, all parties are known to each other".

"Police will continue to work with the families, parents and school of the teenagers involved in this incident going forward," it stated.

'Disciplinary action' taken by high school

The South Australian Education Department said that the school where the students attended was aware of a police investigation into an incident that happened off-site after school on Tuesday.

"The school has reached out to the family and support is being put in place to assist the victim to return to school," the department spokesman said in a statement.

"Appropriate disciplinary action is also being taken.

"A letter has also been sent to the school community outlining the school's response."