Footage of the young girl, taken on the perpetrators' mobile phones, allegedly shows her screaming as eight fellow students pull down their trousers and undersea, as they rub their buttocks and genitalia on her.
The incident occurred in 2011 but it was not reported to the Education Department of South Australian police until April this year, a department spokeswoman said.
READ: TEENAGER REVEALS PHOTOS OF HER RAPE WENT VIRAL: 'EVERYBODY KNOWS'
In April, the girl told a Child and Adolescent Mental Health Service worker and her mother she was being bullied by a girl and a gang of eight boys, culminating in the recorded incident.
Her mother told Adelaide Now: “(The girl) taped my daughter to a tree out the front of the school and staffroom where most students gathered at recess and lunch.
READ: TEENAGE BOYS CHARGED WITH CHILD PORN AFTER FILMING THEMSELVES HAVING SEX WITH 15-YEAR-OLD
“My daughter noticed a gang of boys approaching... she began to ask (the girl) to let her free... as the boys drew closer they grabbed the hose and sprinkler.
“By this stage my daughter was screaming and begging (the girl) to untie her; however, the girl chose to laugh at her and do nothing.”
The woman said the gang of boys wrapped her daughter in the hose and, while filming her with mobile phones, “dropped their pants and rubbed” their buttocks and genitalia on her.
“She said she felt so humiliated and so frightened... everyone around her stood there and did nothing or were laughing in the distance,” she said.
The bullied teen has since left the high school and is continuing her studies elsewhere.
READ: ARE YOU AN ONLINE BULLY? WHY PEOPLE ARE SO QUICK TO JUDGE 'MAGALUF GIRL'
Her mother has said she wants to know why no teacher or adult was aware of the alleged incident, and why her daughter’s “withdrawal” was not followed up by staff.
“I admire my daughter’s strength of character in trying to deal with this alone and also completely understand why she has never spoken out before."
A police spokesman has said he was unable to comment “as the matter is currently under investigation”.