Toumei Ningen - Iregui Chuuzou

The protagonist, Hirumada Chuuzou, a chemistry teacher at the private Narumizawa Academy, has been secretly researching a drug that turn people invisible. However, he is fired after it is discovered that he is using school equipment for his personal research, and his previous sexual harassment behavior has also become an issue. When Hirumada came to pack up his belongings, he drank the newly completed drug, becoming invisible and taking up residence at the school. Taking advantage of the fact that he was invisible, he engaged in lewd acts as revenge against the girls who have made fun of him and the school, and to satisfy his own desires.

[From Getchu]