Katekyou Hitman REBORN!

Nowadays, the mafia world is ruled by Italy's strongest family, the Vongola. Vongola's current head, ninth in the row, is an old man ready for retirement. But before he can hand down his position, he needs help from a professional Hitman to prepare the next generation Boss for ruling over Vongola and the mafia world itself.
Sawada Tsunayoshi is just your ordinary teenage boy who attends Junior High and sucks at it. He is unpopular, has no friends, and is desperately in love with a girl from his class. But his life takes an extraordinary turn when Tsunayoshi meets Reborn, an infamous hitman, who the ninth boss of Vongola has sent to train Tsunayoshi for his upcoming title.
(Source: Katekyo Hitman Reborn! FC)