Kindred Spirits on the Roof

Kokonotsuboshi Girls' Academy of Commerce, a school built on the site of an old castle, nicknamed "Shirojo." One day, the timid Toomi Yuna, who had been living a quiet life at this school, suddenly met two ghosts on the roof during lunch:

Enoki Sachi, who died in an accident before the war, and Nagatani Megumi, who died of illness 30 years ago. The two "kindred spirits" died with unrequited feelings in their hearts, met, and fell in love. They ardently wish to experience their "first time" together before their eternal rest.

...But they don't know how.

The two who are bound to the site of the school enlist Yuna to help them create more "yuri" couples at the school so that they can glean some sexual insight from observing them, and to assist the girls struggling with their hidden feelings.

[From MangaGamer]