Rakuin Mesu Kyoushi (Jap) by Potage - Adult Sex Game

The main character is a 25-year-old slacker working as a substitute teacher.
Two new teachers joined his school this year.
He developed a fondness for one of them, Koimari Nao.
However, he knew that this school was not the best place for young female teachers.
So he decided to protect her from all evil, both inside and outside the school.
But as you know, the road to hell is paved with good intentions,
and he soon became dependent on Nao, believing that platonic love wasn't enough.
He wanted more from the girl, so he decided to act...