A Chester County high school student brought a BB gun into a homeroom class and shot a classmate in the chest with a plastic pellet, police said Friday.

The student was unharmed except for a "minor red mark" on his chest - and school administrators at Westtown's Rustin High School only learned about the incident after another student sent them an anonymous note, Westtown-East Goshen police said in a release.

The 16-year-old student who brought the BB gun to school had been keeping it in his backpack; police said he brought the BB gun to school so he could take it to a friend's house afterward.

In homeroom Thursday morning, other students saw the BB gun in the student's backpack and told him to shoot it at another student, after which the student fired a single pellet at his classmate, police said. Police didn't specify whether a teacher had been present in the room at the time or whether the school went into lockdown.

The owner of the BB gun and the student he shot are friends, police said, and the injured student's parents declined to press assault charges. The student will receive citations for two summary offenses, sale and use of air rifles, and disorderly conduct, police said. He's also facing "disciplinary action" from the school district, police said. - Aubrey Whelan