BOSTON?A 17-year-old girl, who authorities say agreed to take part in a Columbine-style massacre at her school but spilled the secret by warning her favorite teacher that the woman was in danger, was charged Tuesday with conspiracy to commit murder.

Amy Lee Bowman, who pleaded innocent, is the fourth teenager charged in what authorities say was a plan to smuggle guns under black trench coats into the school and emerge from a bathroom shooting to kill.

Three other students?brothers Eric McKeehan, 17, and Michael McKeehan, 15; and Steven Jones, 15?were charged earlier in the plot to kill “thugs, preps and faculty” at New Bedford High School.

They pleaded innocent Monday to conspiracy to commit murder, possession of ammunition and other charges.

An unidentified 16-year-old is scheduled for arraignment Wednesday on the same charges, police said.