I'm not sure if it seinen or shounen but the main character is a girl....

it's an action manga, here the story :

• She(mc) came to this an all-girl high school. The high school is so popular cuz it's build on an island. The reason she came to this school is to find out the cause of her big sister death.

• When she go to class as a new student, all of the girls in the class stare at her with a murderous eyes. She feel something odd about this school. This one girl show her around the school and she(mc) see all the well mannered beautiful girls that walk pass her.

• The girl give her this pin or something (I don't know) because all the other girls got one .
  And when this tower bell start ringing, all the beautiful well mannered girls in the school
  turn into a wild beast and start to beat the crap out of one another to steel each other pin.
  She have to survive everyday in this school with this crazy rules.....

" I know the story well but I forgot the name of this manga.
I didn't finish reading it and want to continue. If someone know
this manga, please tell me the name of it. "

" Thank you ^^ "

I'm pretty sure it's Tokyo Girls Destruction.

Yes ! That's It !
You're Awesome, Thanks ^^