A mother has spoken out about how her daughter, 13, was relentlessly bullied and then gang raped by two older boys before committing suicide two years later.

Cassidy Trevan was forced to miss out on her fourth term of Year 7 at a Melbourne high school after she was targeted by a group of bullying girls.

Her mother Linda told 9 News that the girls would slap her on the face, leave banana peels at the front door of their family home and regularly abuse her on social media.

Cassidy finally returned to school two days a week - where she was met with an apology from the girls, who asked her to be their friend and invited her to a festival.

But instead of going to the festival, the very same girls led her to a nearby house where she was subjected to a horrific gang rape that savaged her innocence.

Linda Trevan has opened up on how her teenage daughter Cassidy (pictured together) took her own life after relentless bullying and a horrific gang rape by her tormentors

Cassidy Trevan was forced to miss out on her fourth term of Year 7 at school after she was targeted by a group of bullying girls

'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 told 9 News.

'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.'

In a heartbreaking open post on Facebook, Ms Trevan said she spent the next two years 'desperately doing everything' she could to keep Cassidy alive.

'I had to watch my baby suffer for the next 22 months from these demons,' she wrote online.

'She worried you would find her and get her again, she went through continued bullying from some of you who managed to get to her by phone or social media, via others, even after what you'd done to her.'

'She suffered flash backs of the crime, nightmares, insomnia, separation anxiety, panic attacks, PTSD and subsequent worsening mental illness.'

After making a recovery and returning to school, the bullying girls apologised to her and asked to be her friend. They asked her to go to a festival, but instead of taking her there she was led to a house where two older boys raped her, her mother claimed

The event caused Cassidy to suffer 'nightmares, insomnia, separation anxiety, panic attacks, PTSD and subsequent worsening mental illness,' Ms Trevan said. After two years of suffering, Cassidy committed suicide aged 15

The suffering teenager moved schools to escape the bullying, but was subjected to further verbal abuse by the main bully online.

'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 told 9 News.

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

But because Cassidy never made a formal statement to Dandenong police, afraid of the social repercussions, authorities were never able to lay any charges.

Tragically, after years of torment, Cassidy committed suicide on December 12, 2015.

Cassidy never made a formal statement to police so they were never able to lay any charges on the alleged rapists

'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,' Ms Trevan wrote.

'What you did to her was a direct cause of her suicide on 12th December 2015.'

'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.'

'I'm not a mean, angry, or vindictive person ... but what you kids did ... I hope you never forgive yourselves and never forget the name Cassidy Trevan. You all have blood on your hands for as long as you live.'

'Bullying killed my child, bullying must be taken seriously.'

'The death of any young person is an absolute tragedy and our sympathies are with Cassidy's family,' a spokesman for the Victorian Department of Education told Daily Mail Australia in a statement.

'Schools have a range of ways to help students who may be experiencing bullying or mental health issues, including by providing qualified counsellors. School staff work hard to identify and support students who need support and we would encourage any students who need help to talk to staff at their school.'

Daily Mail Australia has contacted Cassidy's school and Linda Trevan for comment.

For confidential support call the Lifeline 24-hour crisis support on 13 11 14.

Anyone across Australia experiencing a personal crisis or thinking about suicide can contact Lifeline.

Regardless of age, gender, ethnicity, religion or sexual orientation their trained volunteers are ready to listen, provide support and referrals.