A Victorian mother has told of how her 15-year-old daughter committed suicide after a year of schoolyard bullying which culminated in the main perpetrator orchestrating her gang rape.

Cassidy Trevan was in Year 7 at a public high school in Melbourne’s south-east when she was set upon and bullied relentlessly by a group of girls, her mother Linda Trevan told Nine.com.au.

“The bullying started out with just insults and stuff and she got slapped across the face once or twice,” she said.

As well as waging their war on social media, the girls would follow Cassidy around at the local shops.

“Our fence got graffitied, banana peel on the front door mat,” she said.

The bullying became so bad Cassidy missed her entire fourth term at school in 2013.

After taking part in a recovery program, her daughter finally felt strong enough to return to school for two days a week in February 2014.

The girls who had bullied Cassidy said they were sorry, apologised to her and asked her to be their friend, inviting her to join them at the Springvale festival the next day.

It was there that her new "friends" executed a cruel and calculated act of betrayal, leading her to a nearby house where they had organised for her to be gang raped.

“They were older boys that Cass didn't know. Two girls who sat and waited. Two boys who shared her and timed each other. One boy stood guarding the front door,” Ms Trevan said.

Cassidy was only 13 years old.

Cassidy's mother has shared the harrowing story of her daughter's suicide to warn that bullying costs lives. (Photo: Linda Trevan/Nine.com.au)

Cassidy reported the gang rape to Dandenong police and spoke with detectives several times, but she was never able to bring herself to make a formal statement.

“Cass was scared to make a formal statement for fear of retaliation from the gang, and she also was worried reliving it would ‘push her over the edge’,” she said.

Ms Trevan said police told her they couldn’t do anything to bring the perpetrators to justice because “there's ‘no victim’”.

Cassidy had begged her mother not to go to the school principal about the bullying before the rape, out of fear it would get worse.

After the rape Ms Trevan did go to the school but was told as it was a sexual assault it was now a police matter.

Nine.com.au has contacted Victoria’s Department of Education and Training for comment.

Detectives from Victoria Police's Sexual Offence and Child Abuse Investigation Team (SOCIT) met with Cassidy and her mother more than 20 times over a two-year-period.

But they were ultimately forced to drop the case because of a lack of evidence and a victim statement, however counselling and victim support was offered.

"During those conversations the SOCIT detectives explained options, put protections in place and ensured the victim was receiving assistance from support services," a Victoria Police spokesperson said in a statement.

"They encouraged the victim in each interaction to make a statement.

"Throughout this time period the SOCIT followed up some other investigative leads.

"The victim declined at each stage to proceed with the matter. No charges were laid."

Ms Trevan tried in vain to resettle Cassidy in two new schools but her daughter was suffering from flash backs of the crime, panic attacks and PTSD.

In the three and a half months before Ms Trevan managed to move homes the bullies hounded Cassidy relentlessly, contacting her on social media and on her phone.

“I had to get an intervention order on the main bully girl when she physically assaulted Cass at the shops, after the rape, and she was even calling my mobile demanding to talk to Cass,” Ms Trevan said.

Laying bare her suffering publicly for the first time, and pointing the finger squarely at Cassidy’s bullies, Ms Tevan wrote in a harrowing Facebook post on January 29 that Cassidy never got over what her bullies had subjected her to.

Her beloved daughter’s suicide in December 2015 was a direct result of all she had endured, she wrote.

“I helplessly watched my precious child wither away before my eyes, mentally & physically, until she rarely got out of bed, until she could no longer take the pain and torment you caused her,” she wrote in the post.

“I know who you are, you know who you are, and the police know who you are. I hope the knowledge of what you did haunts you for the rest of your lives, and one day, if you are lucky enough to have children of your own ... remember what you did to my precious only baby, and imagine how you'd feel if someone did that to your baby.

“Cassy was my world, she still is and she always will be. But now I have nothing, and I'm still trying to find a reason to go on without her.”

Ms Trevan said she was coming forward to tell her story now because she wanted to send a clear message to bullies.

“I want the bullies out there to know that it’s not just a game. It costs lives.”

Readers seeking support and information about suicide prevention can contact Lifeline on 13 11 14.