She told the court she was walking home from school when she saw four youths surrounding a young boy.
She said: "I thought they were just robbing someone."
The witness named three of the teenage boys facing trial for Damilola's murder. She said she knew the fourth boy by sight.
The court heard that although she was the only witness to the attack, she didn't see who stabbed Damilola.
The girl said she didn't want to go up to him in case she got into trouble. She also saw the boys pass round a bottle, the jury heard.