From MangaHelpers:

Nadeshiko Otemachi’s entire family are killed in a terrorist attack… A few years later, she enters into a military school for the sake of protecting Neo Tokyo. In order to get out of an awkward situation though, Nadeshiko decides to join the S Class, a specialist course for the most elite students in the school. But the S Class, and its S rank members, isn't exactly what she expects. [Niffer]