家庭教師ヒットマンREBORN! - 3年M組獄寺先生

Sawada Tsunayoshi is regular boy who has never excelled in sports, school or social life, thus being named "No Good Tsuna". Everyday, he leads just a normal life, living with his mother, while his father is away for work. But the thing Tsunayoshi doesn't know is that his family is actual lineage of the great Mafia Boss, the first head of Vongola, Giotto.
Because of the Sawada history, Tsuna has been chosen to become tenth head of the Vongola family, an infamous Italian mafia family which rules the Mafia world. To become worthy of the title, Tsuna is being aided by professional hitman and infant, Reborn, who takes the role of Tsunayoshi's mafia tutor.
(Source: Katekyo Hitman Reborn! FC)

Note: Chapter count includes two extra collections of short chapters published in volume 42

Pairing: Yamamoto x Gokudera - 24!80x14!59

Gokudera thinks his teacher and lover Yamamoto doesn't take him seriously because they've only had sex twice in over half a year.

V1: 3-nen S-gumi Yamamoto Sensei   
V2: 3-nen S-gumi Yamamoto Sensei R