The emotional letter a teenage girl wrote to her classmates after she was gang raped.

Warning: This post addresses issues of suicide and sexual assault. It may be triggering for some readers.

Cassidy Trevan was only 13 years old when two girls who had bullied her at her local public high school in Melbourne’s south-east organised for her to be gang raped.

The girls led her to a nearby house where two older boys, that Cass didn’t know, took turns raping her.

Twenty-two months later, Cassidy took her own life.

Now, her mother is sharing her story - including the heartbreaking letter Cassidy drafted to warn her 1500 fellow students about the dangers of bullying.

The unfinished letter was found on her laptop by her mother, Linda Trevan, who shared it with Nine.com.au.

"I was a student at [school name withheld] and I was raped by some of the students who still attend that school," Cassidy wrote.

"My aim is to warn other people...about what happened because I’m worried if they could do it to me they could do it to other kids like me, or at least try to.

"I’m not doing this for revenge to those students that raped me, set up the rape, bullied me about the rape, teased me about the rape or anything like that. I’m also not doing this for attention seeking like I mentioned earlier, I want to make that clear."

Cassidy also wrote that she was writing the letter for herself, in hopes that she would "finally be left alone".

"I still continue to get students I’ve never met contacting me on Facebook calling me a slut. I’ve moved schools, I’ve moved house and still I’m being contacted and bullied. I can’t stop people from spreading rumours, but at least I could try and spread what really happened.

"My name is Cassidy Trevan, and I was raped," she wrote.

Cassidy's letter ends with words of advice for anyone who might find themselves in a similar situation.

"If anyone ever tries this on you trust me it’s worth fighting! Fight! If you don’t you’ll regret it for the rest of your life like I do. You can do it," she said.

Cassidy's mother, Linda, told Nine.com.au the bullying started when her daughter was in Year 7.

"The bullying started out with just insults and stuff and she got slapped across the face once or twice...Our fence got graffitied, banana peel on the front door mat," she said.

Cassidy was forced to miss her entire fourth term of school, but when she returned to school for two days a week the following year, the girls apologised.

Linda said the girls asked Cassidy to be their friend and invited her to join them at a festival the next day. Instead, they took her to a nearby house where they had organised for her to be raped.

Cassidy reported the attack to the police, but never made a formal statement for fear of retaliation.

For 22 months, Cassidy suffered from panic attacks, PTSD and flashbacks of the crime. The girls continued to bully her after Cassidy changed schools numerous times.

"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," Linda Trevan said.

Cassidy took her own life in December 2015.

"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," Linda wrote in a Facebook post directed at her daughter's bullies on January 29.

"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."

If you or anyone you know needs help, Mamamia urges you to call Lifeline on 13 11 14. If you're experiencing sexual assault or domestic violence, call 1800 RESPECT on 1800 737 732.