マテリアルポルカ

From MangaHelpers:

Alice and Ruka are heading to an exclusive girl's boarding school together! They realize they may be in over their heads when they arrive at the opening ceremony, where they see that there are markedly less students in each increasing year. Their school dreams turn to nightmares when the ceremony concludes with the student council president executing a student for wearing a slightly too short skirt. Her shattered head explodes into sweets. One thing follows another on that terrifying first day and night, as friendships are betrayed, new allies are found, and Alice obtains a mysterious chainsaw named "Ibara-hime." Will Alice be able to survive her new school life? [tethysdust]

Alternate summary by the scanlator: "Lesbian school girls murder each other with deadly weapons and explode into candy."