姦禁～18の穴・もう注ぎこまないで！～

Main character lives happily surrounded by classmate Haruka, tutor Mizuho and homeroom teacher Mizuhime-sensei when he receives a strange letter from father informing about sister arrival. The next day mysterious girl Sao gets caught up in strange incident. Then anonymous letter comes saying only "I'm waiting for you at school". Protagonist waits at school till night, but when he goes to investigate light and screams in the classroom, he gets hit in the head and loses consciousness. On awakening strange men surround him and demand to train two of girls. To his surprise, girls are Haruka, Mizuho, Mizuhime-sensei and Sao...

[From vndbreview]