Kotsutsubo (2012)

Eri is an inconspicuous female high school student who goes completely unnoticed by her classmates. In contrast, her childhood friend Mitsuko is the class leader and is constantly hit on by their creepy teacher, Chida. In order to protect her friend, Eri obtains a funerary rumored to hold ashes that cause certain death for anyone who gets them in their mouth.