Stephanie Torres pleaded guilty to aggravated assault for the June 25 attack.
Police say Torres found the 12-year-old erasing a comment on her MySpace Web page and used an inch-long folding knife from her keychain to stab the girl three times.
The girl had wounds to her neck, arm and rib.
Torres, who has been charged twice in the past with assault, faces anything between probation and five years in prison when sentenced next month.
Information from: The Star-Ledger, http://www.nj.com/starledger
