The Anime Lad
Demis have lived beside humans for ages and have only recently gained broader acceptance. A high school biology teacher with a strong interest in them meets several at his school. They include a vampire, a dullahan, and a succubus. His curiosity now has to make room for the ordinary people behind those remarkable labels.
Super Eyepatch Fox
Take familiar creatures from folklore and place them in a modern school as people called demis. Give each one her own everyday concerns. Then introduce a biology teacher who wants to understand. Interviews with Monster Girls builds its premise from conversation, with old myths meeting the less dramatic reality of sharing classrooms and hallways.
Gigguku
A biology teacher meets actual demis at his school after years of interest, and suddenly every old monster label has a person answering back. I love that starting point. A vampire, a dullahan, and a succubus may sound like a supernatural roll call, but the setup promises individual problems rather than one tidy explanation for everyone.
Father's Basement
A teacher fascinated by demi students could become uncomfortable very quickly, especially when the girls are introduced through familiar monster categories. The premise works best at human scale. These are people living in a society that only recently began accepting them, and each arrives with her own concerns rather than serving as a folklore exhibit.