CCTV footage was shown of the attack, in which two girls can be seen hitting the victim repeatedly.
The defendant, who cannot be named due to her age, is then seen to join in and hitting the victim on the back of her head.
She admitted assault and told the Youth Court: “I feel very bad about what happened. I’m very sorry.”
Magistrate Bridget Shaw told her: “It was a very nasty incident and you can have no excuse for getting involved.”
She was subsequently a six-month probation order and a restraining order.
The restraining order will ban her from direct or indirect contact with the victim, following or approaching her or loitering near her home, and will last as long as both girls are at school.