Michael Pace, 16, pleaded guilty on Friday to two counts of first-degree murder and two counts of aggravated battery just as his trial was getting underway. Jury selection was slated to begin.
Holt, 16, was on his way home from Julian High School, where he was an honor student, when he was shot and killed while trying to protect a friend on the CTA bus.
Five other people were wounded.
Prosecutors say Pace pulled the trigger. Another teenager, 15-year-old Kevin Jones, pleaded guilty to murder conspiracy and was sentenced to 10 years in prison for providing the gun.
Holt was not the intended target. Pace was going after a rival gang member when he opened fire on the bus, police said.
Jones mother, Karin, released a statement: I'm sorry about everything that [h]as happened. Sorry for the whole family, what happened to their son. And I'm sorry for my son, what has happened. I just want to apologize for my son and to the family and just stop the violence out here in this world.