A teenage boy has been charged after allegedly firing an air gun, plunging a Sydney school into lockdown.

Rose Bay Secondary College in Dover Heights, in the city’s east, was placed into lockdown about 1.45pm on Thursday after reports a student had been seen with a firearm.

WATCH THE VIDEO ABOVE: High school student arrested over shooting with BB gun in Rose Bay

Know the news with the 7NEWS app: Download today

Police allege a 15-year-old boy showed another student what appeared to be a gun before firing a pellet at him.

The projectile, described as a “BB gun”, reportedly struck the student in the back, though police said no injuries were reported.

A teenage boy has been charged after allegedly firing an air gun at a school. Credit: 7NEWS

The school was plunged into lockdown over the incident. Credit: 7NEWS

It’s understood the weapon was a shotgun-style air gun, capable of appearing highly realistic, which heightened concerns when initial reports were made to authorities.

Officers arrested the boy at the scene and seized a ball-bearing firearm believed to have been used in the incident.

He has been charged with possessing an unauthorised pistol in a public place, stalking/intimidating with intent to cause fear of physical harm, and firing a firearm in or near a public place.

The boy was refused bail and is scheduled to appear before a children’s court on Friday.

It’s understood the weapon was a shotgun-style air gun, the kind that can look very realistic. Credit: 7NEWS