Boku Igai no Otoko o Shiranai Kanojo ga Hoka no Otoko ni Dakareteita -Kyouiku Jisshuu Saki de Netorareru Kanojo-

Yoshiyuki’s girlfriend, Shiho, returned to her alma mater as a student teacher.
There, she reunited with her senior, Murase, whom she had admired during her school days—now a fellow teacher.
Faced with the person she once looked up to, Shiho couldn’t help but feel giddy.
Murase, seeing right through her feelings, began to take advantage of his position as her supervising teacher to get closer to the now beautifully matured Shiho.
Before long, Yoshiyuki started noticing something off about Shiho.
She stopped talking about school, avoided making time for the two of them, and gradually became less attentive toward him.
Yoshiyuki’s unease grew when his younger sister, Momoka, who attended the school where Shiho was teaching, told him about Shiho’s behavior there.
Then, one day, while visiting the school for work, Yoshiyuki witnessed something shocking…

[Translated from dlsite]