Destro 246
Genre: Action, Mature, School Life, Seinen
Author: Keitarou Takahashi
Artist: Keitarou Takahashi
Serialized in: Sunday GX
Publisher: Shogakukan
Summary: Billionaire Tono Ryuichi, the President of Tono Foods Group, a giant food company, recently acquired two young female assassins from the Mexican cartel. Sui and Ai came from unimaginably poor childhoods and were given a choice between killing people or selling their body. They chose the former and went through an inconceivably extreme training and, at 17 years old, they have become one of the deadliest assassins in the world. They are absolutely obedient to their master and they are given the task of finding the killer that poisoned Ryuichi’s wife and child. They begin attending high school to blend into society and, at the same time, read and study which helps them become even more deadly on the field. Sui and Ai encounter Matoba Imari who is a high school assassin herself and may be more skilled than both of them. But unlike the two, Imari just wants to lead a normal life. Whatever Imari wishes for her life is not their problem, because the only things Sui and Ai feed on are bloodthirst and a challenge. [vyc]

Currently my favorite series, 36 chapters (ongoing) in Japan, 9 in scans.
Warning: the series pretty much features only women, all the men that appear in it somehow get killed off afterwards so if you don't want to read a series that focuses on women, not for you.