Bystanders stood by and filmed the incident, which started with a comment about English class.
The video shows a teenager taunting the young girl, before throwing several punches.
The young girl was taken to Logan Hospital with a concussion, swelling, and cuts.
“It was quite confronting and disgusting and you know to be a parent and think your child can go on public transport and has always gone on public transport,” the teen’s mum said.
“Yeah, it just makes you sick to the guts.
“She had concussion, she had swelling to the face, cuts in her mouth.”
The woman said a statement was given to police who said her daughter’s attacker would “have the book thrown at them”.
The alleged offender was given a warning by police, with no charges laid.
The mum believes a girl at her daughter’s school set up the attack.
She said had the offender been an adult, they would have been charged.
“There would be justice served and it turned out that the justice system – it’s just a caution,” she said.
“As a parent, it’s hard to comprehend that someone could do such an act of violence and that’s all they get.”
Queensland Police confirmed with 9News that they investigated the incident.
In a statement, police said they’d taken a “diversion first” approach, which is a requirement under the Youth Justice Act.
“The next person might not be so lucky and that’s what needs to stop: all these one-punches, all this assaulting people. There’s no need for it,” the mum said.
“Physical scars heal but emotional ones don’t and there’s no timeline – like this could be with me forever and it’s definitely impacted me as a parent.”