Saitama Chainsaw Shoujo

Kirisaki Fumio's just a highschool girl in love...

...but then she finds out that the guy she's fallen for happens to love someone else.

...so she decides to go out and get a chainsaw.

And take it to school.

And then many things happen.

(Source: Horobi no Michi)

Saitama Chainsaw Shoujo is based on Hiroshi Sakurazaka's 2004 short story of the same title, which won 16th SF Magazine Readers Award.