This is a strict cool teacher Ahibutiuchi! / Genkaku Cool na Sensei ga Aheboteochi!

This is the first of 2 episodes. The director of this "creation" was Ishige Rui, who directed The Knight in Snow-White Panties (2014). The main character of the hentai, Souma, has long been in love with his older sister Rin, but after childhood tenderness, they grew up and she cooled towards him (according to the hero). However, she works at the school where Souma studies, and finally gathering his courage, he sent her a love letter...