The mob attacked 16-year-old Sophie Russell, punching her in the face 20 times.
Her injuries were so severe that she had to have facial surgery and take 10 months off school to recover from the trauma.
But the teenage gang of four girls and five boys were let off after police only cautioned one of them.
Sophie, who has Asperger's syndrome, said: "What I went through was really traumatic and ripped the family to bits. I was just walking to my boyfriend's house and wasn't expecting to be attacked.
"I had to jump over a wall to get away from them, then I called my Dad and he came and took me straight to hospital."
Her mum, Helen, 45, has criticised police for not prosecuting the youths. "I am absolutely devastated," she said.
"If they are doing that at the age of 15 what are they going to be doing at the age of 25?
"There is no justice. They have got away unpunished, but it will always be at the back of Sophie's mind. That is terrible.
"To think someone could do that to another person is horrific. She is scarred for life. It will be seen on wedding photos."
The attack happened at 5.30pm at Sophie's school in Louth, Lincolnshire, in February. She was discharged that evening but found out four weeks later her nose was in fact broken and had to have surgery.
She has since moved to a new school in Lincoln and despite missing most of year ten is on track to take her GCSEs at the same time as everyone else.
She has now been chosen by the Olympic Games committee to be a 2012 torch bearer.
Sophie added: "I want to show everyone they can turn their life around no matter what has happened to them."
A Lincolnshire Police spokeswoman said the case was passed to the Youth Offending Team who decided not to press charges but to give a caution for common assault.
She said: "Age is a factor in the decision but everything would have been taken into account."