Senkou no Rumina Zenpen ~Sennyuu! Sei Alucard Jogakuen~

Golden week was over, and humid nights marked the arrival of summer.
A lone student ran through the bamboo grove of St. Alucard Girls Academy as if fleeing for her life.
But, as she broke from the grove off the mountain dormitory campus and onto the road of freedom, the student body leader and several others waited in ambush.
She begged for them to let her go. She knew the truth about this place and only wanted to go home, but they seized her and escorted her back inside.

THREE MONTHS LATER
It was late August, and detective Rumina Shingu was wrapping up an investigation in Shinjuku.
Rumina operated under her own name as a private investigator, but in fact she was trained and dispatched by a large organization. Her specialty was to deal with unusual cases.
Her assistant and effeminate little brother, Alice, already had news of her next assignment.
They were to sneak into St. Alucard Girls Academy.
Not one to question the mechanisms of upper command, Rumina set off with newly recruited agents Ayako and Kirika, and crossdressed brother Alice, all posing as second-year transfer students.
The day before the mission they arrived by car, separated by an hour or so, at the nearby City Hotel.
While the team was meant to work together, Rumina and Ayako clashed like rivals.
The mission was off to a rough start. Into the dreaded campus they go...

[From DLsite English]