I can't remember the title of this manga. I read only the first chapter
Yakuza girl


  Senguu Fumihiro is a young boy that made a promise to his dying grandmother to find a good wife for himself. With this intention he enrolls in a prestigious school but on the first day his world turns upside down when all the students begin to kill each other and one of them even turns into a monster. He is saved by a girl named "Akari", sent to protect him by one of the factions that rule the school.