Jonason ~Kugutsu no Mai~

Kamome and Arisa are two freelance detectives, and their current assignment is to protect Kimiko Ichijou, the daughter of a wealthy oligarch. Three days before her birthday, someone tipped the detective off about a possible assassination attempt. Kimiko has been recently transferred to the prestigious Seika Girls High School. Kamome decides to infiltrate the school, working undercover under the disguise of a janitor. Soon he realizes that powerful figures from a corporation that Kimiko's family is part of are not about to give up, and his life is in danger as well...

[From Moby Games]