Shanni, now aged 13, squared up to the girl and punched her again and again, so powerfully that the girl's head slammed into a wall, Sheffield Crown Court was told.
A teacher who witnessed the attack - which was watched by a mob of 100 fellow pupils - described it as "vicious".
The next day, the court heard, the girl retaliated, and attacked Shanni in an English class in revenge.
But the 13-year-old - who cannot be named because of her age - later apologised to Shanni and said she had wanted to scare her but hadn't intended to seriously injure her.
She said: "I scratched her with the blade. I meant to show her, 'If you come to me again I will do that'. I want to put her in the face to show her and scare her."
The youngster, who is not a British national and whose parents were both killed before she moved to England, told the court she didn't like her Sheffield school.
Psychologist Stuart Taylor said the girl had experienced systematic bullying, teasing and mocking about her dress, her language, her learning difficulties, and whether she had a right to be in the UK.
Mr Taylor said in the girl's native culture conflict between women was often resolved by scratching the face with the fingernails.
He said: "Frequently women pick up an artefact, a stick or stone for example, and the face is always the target. The wound normally heals in a month with no visible scars."
The court heard the girl and Shanni, who was 12 at the time of the attack last October, had fought the day before.
A teacher who saw the playground fight from a classroom window said 100 pupils had gathered to watch.
She said: "Shanni was standing in front of her with clenched fists. She was standing square like a boxer. The girl was punched again and again, so hard it slammed her head into the wall.
"I was looking on in absolute horror. It was vicious. I couldn't believe what I had seen."
In a police interview read to the court, the defendant told police she was hungry because she had been fasting for the Muslim festival of Ramadan.
She admitted she had 'scratched' her classmate in the face in revenge for the punches to her face..
She denied pre-planning the attack on Shanni, adding she only took the blade out of her pocket when she saw Shanni in the classroom. In police interview the girl said: "I wanted a little bit scratch her face for what she did to me. I couldn't scratch her with my nails because I cut my nails."
The trial continues.