家庭教師ヒットマンREBORN! - 3年S組山本先生R

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 - 14!80 x 24!59

V1: *3 Nen M Gumi Gokudera Sensei*  
Yamamoto is uneasy because although he and his teacher Gokudera are in a relationship, he fears his more experienced lover is playing with him.

This is probably a play on the Japanese television drama "3 nen B gumi Kinpachi Sensei".

V2: *3 Nen M Gumi Gokudera Sensei R*