At her trial in July, the court heard how the teenager had pulled the comb from her handbag and struck Julie, with the tip of the comb penetrating deep into her brain.
A replica of the black comb with a spiked metal handle had been shown to the jury.
A witness saw the defendant strike Julie's collarbone without much effect and then plunge the comb into her skull.
After Douglas was arrested, she was found to have left a BlackBerry message saying: "I see some girl that I hate, like I actually hate her with a passion, and I kind of stabbed her
Judge Nicholas Cooke warned parents to be aware of the dangers of young girls wearing such combs as "fashion accessories".
"We heard evidence that a pintail comb
can be used as a lethal weapon," he said.
"We heard it can be worn in the hair as a fashion accessory
"It can be as effective a killing instrument as a stiletto knife
. It is not a very nice thing to have in your hair."