The boy was arrested after medical examination of both the victim and the accused.
The incident took place at Begunia village under Simulia police station yesterday. The victim's mother, who is a school teacher, lodged a complaint before the police.
According to the complaint, when the victim's parents were not in the house and the girl was playing outside, the boy, who is a student of standard X and known to the victim, called her inside and sexually assaulted her, the police said.