Sarah Kolb, 17, faces up to 60 years in prison.
The victim, Adrianne Reynolds, had just moved to East Moline from Texas about two months before she was killed. Prosecutors said she was just trying to fit in at a new school but picked the wrong friend.
On Jan. 21, 2005, Kolb, Reynolds and schoolmate Cory Gregory were in Kolb's car at a fast-food restaurant when the fight began. Reynolds was killed and her body was burned, dismembered and hidden in two counties.
Gregory also is charged with murder and concealing a homicide. He is scheduled to stand trial May 1.
Kolb showed no reaction as the verdict was read. Her family and Reynolds' family wept quietly as the jury was polled.
The trial was Kolb's second in three months. The first ended in a mistrial in November when a Rock Island County jury deadlocked 11-1 in favor of conviction. Her retrial was moved to Dixon, about 60 miles from the Quad Cities.
In the retrial, Rock Island County State's Attorney Jeff Terronez said Kolb wrote in a class journal that she was going to kill Reynolds just hours before Reynolds was beaten and strangled in the car.
Terronez said Kolb was angry because Reynolds had shown interest in Kolb's boyfriend and Gregory, her ex-boyfriend.
Defense attorney David Hoffman told jurors it was Gregory who killed Reynolds. But prosecutors said Kolb was still accountable because she choked and beat Reynolds before Gregory "finished her off."
Prosecutors say the two took the girl's body to Kolb's grandparents' farm and burned it, then returned two days later, sawed the body into pieces and dumped the remains on the farm and in Black Hawk State Park in Rock Island.
In her first trial, Kolb testified that Gregory strangled Reynolds, then hit Kolb and threatened to kill her, her family and her cats if she reported the crime. Gregory, who has pleaded not guilty, denied that account in a television interview.
Kolb did not testify in her second trial, and the defense rested without presenting a case.