Wonder School Boy

Tsubaki Tenji seems like a normal high school boy, but at night he secretly works as a detective! Though he pretends to be kind of dim and clumsy at school, he's very physically skilled, and he fights using 500 yen coins as projectiles. However, he's not working just because he wants to-- he's got to pay off his vanished father's ridiculously large debt. He works for the Muroto Investigative Agency, under Muroto Shino, and alongside Shino's teenage sister, Yuki. He's always sleepy during the day from working at night, but if the school finds out the truth he could end up expelled.

(Source: MangaHelpers)