Buso Renkin Vol 5 (Manga)

Talk about being at the wrong place at the wrong time! High School student Kazuki Muto had no clue what he was in for when he rescued damsel in distress Tokiko from a monster known as a "homunculus." Disguised as humans - who actually eat humans - homunculi are malevolent creatures that affix themselves to people's brains, and once fully grown, the only thing that can annihilate them is a weapon called Buso Renkin!

Upon arriving at the hidden headquarters of the League of Extraordinary Elects (the L.X.E.), the Alchemist Warriors learn of the League's plot to attack the high school. Kazuki and Tokiko immediately head over there, only to discover that the school has been surrounded by Doctor Butterfly's newly created humanoid homunculus soldiers. To the Alchemist Warriors' surprise and dismay, they find that not only are they facing Doctor Butterfly's minions, but the students of the school as well!