Tenshi-tachi no Gogo IV -Yuuko-

The Catholic high school for girls, St. Sophia, has a reputation of a very traditional establishment with strict rules. However, there are also rumors that after midnight, something mysterious happens to the school, and everything inside it changes... When the protagonist, a substitute teacher, arrives at St. Sophia for a three months teaching job, he quickly discovers that the behavior of the school inhabitants is anything but compatible with the Christian morals...

[From Moby Games]