Kids Joker

Meet Hotaru Yanagawa: defender of justice, protector of the innocent and high-school student! She's looking to jumpstart her future career as a police officer between homework and home ec., but the sins of the city might be more than she can handle. From kidnappings to crime syndicates, Hotaru will skip the badge and hit the beat in the crime-fighting adventures of a girl detective!

(from mangaupdates)