SSG: Meimon Danshikou Keppuroku

For her brother's sake Otowa, Subaru decides to take his place at an all male prestigious high school for a year until he recovers from being in the hospital. She gets to the school and runs into a few strange guys on her first day, but as soon as she tells them her name—they are surprised and make her go to the Lion Club. Hiding the fact that she's a girl will be difficult enough and now she has to deal with this strange group who are forcing her to join their club because they all believe that in their past lives they were members of the Shinsengumi!

(Source: LuffyNoTomo)