Juuzou Ougami - 大狼十三

A 39-year-old legendary assassin, stung by a mysterious wasp, is transformed into a 13 year-old. Under orders from the Boss of the assassination organization, he attends a Middle School. Though initially bewildered by his sudden school life, he soon finds himself engrossed in the joys of learning. His marksmanship is top-notch.