Keaton West, 16, had a huge chunk of flesh ripped from her leg after the animal escaped and set upon her, a court heard.
Keaton's mother Rachel West and her stepfather were also bitten in the incident in Sale, Greater Manchester, which will leave the teenager scarred for life.
However, her family said they feared it might be much worse.
The dog's owner, Jessica Hinds, 36, also from Sale, pleaded guilty to having the dog, described as a Boxer, dangerously out of control, at Manchester Magistrates' Court on Tuesday.
Mrs West, 46, ran out of the family home at around 10.30pm after hearing her daughter screaming during the attack last August.
She saw the dog 'locked onto' her daughter's leg and along with her boyfriend Mark Jarman began desperately trying to free her.
However, the animal turned on the pair, biting both of them, and only gave up once it had bitten a chunk out of Keaton's left calf.
Mrs West said: 'It was absolutely awful. It was just going crazy and was completely locked onto her leg and wouldn't let go.
'We were trying everything. I even put my hands in its jaw. I was petrified. It was literally trying to drag her leg from under her and get her down on the floor. If it had gone for her face or neck, she could have died.'
Keaton, a business student, required skin graft surgery, spent two days in hospital and magistrates were told she has needed more than 20 further visits after some of her wounds became infected.
Her mother, who also had wounds to her legs and hands, said her daughter will live with the after-effects forever.
She said: 'She was completely traumatized afterwards, she didn't want to go out and is still terrified of dogs. She missed loads of school during her GCSE year because of all the hospital treatment - and, of course, she's still got this dent in her leg.
She's still under the care of the plastic surgeon and they are going to review in a year or two. But whatever sentence the court hands out, she will live with this forever.'
The animal has been kept for the past year in kennels by police. However, the court was told that immediately after the incident, Hinds [had the dog] put down (euthanized).
Matt Wallace, defense attorney for Ms. Hinds, said the incident 'came out of nowhere' and the dog had never shown any aggression before. [surprised owner]
'She accepts this is a serious matter and she has significant remorse. She knows the family and has offered all the help and support she can.'
