Mission! School(2009)

Mission! School

Ishikawa Takuya, a high school student who doesn't stand out in any particular way, is surprised when his principal asks him to take care of some transfer students. He is even more surprised to find that there are actually 30 charming female transfer students in his care, who have come from a boarding school that was destroyed in a fire! At first it seems like a dream come true, but then he notices their knives... and guns... what kind of school did these girls come from?

(Source: MangaHelpers)