Nantai Prima Onna Kyoushi

The main character, Shuichi, lazily spends his days without much to say.

Aika Shirasagi, a Japanese language teacher, is teaching his class with a kind smile on her face. Aika's pride is hurt by Shuichi, and before long she finds herself unable to resist his demands, which escalate each time she meets him.

However, her body, becomes more submissive than her mind when she plays with the protagonist, and she finds herself deep into the depths of her anomalous behavior, even as she worries about her feelings.

Shuichi is also absorbed in Aika's body, which is so flexible that it seems to open up endlessly. Their day-and-night activities gradually sublimate into a passionate and burning ritual.

[From official site]