Hitozuma Onna Kyoushi Reika ~Nyuujoku no Mesu Dorei~

Murakami Toshiaki had a crush for his beautiful homeroom teacher, Tachibana Reika. As his feelings grow stronger, he musters the courage to hand her a love letter, but it was a mistake. Reika tore up the letter after reading it loud in front of all the class's students. Since then his school life became worse, until one day of his 2nd year, he found Reika masturbating on the school toilet. Taking pictures of the shocking scene with his phone, he begins to threaten Reika with those pictures.

[Edited from DMM]