Cops have busted the teenage brother of an 8-year-old boy who was caught with a loaded gun inside his backpack in a Brooklyn schoolyard Friday.

The 13-year-old, who police say is the owner of the gun, was charged as a juvenile with criminal possession of a weapon.

Cops withheld his name because he is a minor.

The younger boy, who has also not been identified, was at PS 91 in Prospect-Lefferts Gardens at noon, when a teacher became suspicious and told him to put the bag on the ground and sit on it, police said.

When he refused, she picked it up, realized it was heavier than it should be and searched it.

She found a pistol inside and called cops.

Police said it was loaded with four bullets, two of them hollow points.

Cops would not say how the gun ended up in the backpack.