So I'm going to give you a summary!
That happened in June 2004 in one of Japan's primary school.
The right one was the victim and the left one was the assailant
The assailant's name was Natsumi Tsuji, she lured the other girl on the school's rooftop
and she cut her up with a retractable knife
This scandal was called the Nevada-tan's scandal
The reason it was called like that is because the assailant was always wearing a hoodie with "Nevada" written on it.
What's even more goosebumps inducing is the fact that she posted a post on her blog saying "Today I was about to kill you, but I couldn't. Tomorrow, I'm going to cut you up with my retractable knife."
What's even more shocking is that the assailant even had a fancafe with fanarts of her
Read more: here (not a really trustworthy source but that's all we could find :s)
original post: here