SCHENECTADY — Two teenagers were arrested on misdemeanor assault, endangerment and mischief charges on Wednesday after they fired an air gun at a school bus, shattering a window and striking a 9-year-old boy who was standing outside the bus, police said.

Three young children were on the bus when the window was shattered but were not injured, police said.

Police converged on the area of Craig Street and Lincoln Avenue just after 8 a.m. and said they didn’t have to look hard for their suspects. They followed the sound.

Johnny Chaparro, 17, of 903 Lincoln Ave., and an unidentified 15-year-old were continuing to shoot in the backyard of Chapparo’s home, police said.

Two weapons were confiscated, one a BB gun and the other a “high performance” pellet gun. Authorities were not sure which was used in the incident. Both weapons use air pressure to propel projectiles, the pellet type larger and heavier than the BB.

No one was seriously hurt. The three preschoolers on the bus were not near the window that shattered.

A second bus was sent to the scene and finished the route to Fulton Early Education Center on Eleanor Street.

The boy who was struck had redness and swelling on his right leg, according to police department spokesman Lt. Brian Kilcullen. The boy was treated at Ellis Hospital and released.

School officials contacted the parents of the children on the bus. They also sent out letters to parents, informing them of what happened.

Chaparro faces several misdemeanor charges, including third-degree assault, second-degree reckless endangerment, fourth-degree criminal mischief, endangering the welfare of a child and possessing an air gun.

The 15-year-old faces similar charges. His case was sent to Family Court.

Chaparro was arraigned and ordered held on $1,000 bail.

Chaparro was also charged last month with third-degree assault and child endangerment after he allegedly punched a 15-year-old in the nose on Lincoln Avenue, according to papers filed in court then.

Bus windows have also been broken out before. In December 2003, a bus at Central Park Middle School had a window broken by an unknown object. That bus was carrying five students. An aide was hit by glass but did not require treatment.