Episode 6 preview is already up on the site:
http://www.anohana.jp/story/06.html
Great to see Jintan going back to school! But I'm wondering (and worried) for whom this note might be directed to, it's shown on the preview at the end of ep5. It says something on the lines of "Die!", I think.
Anaru? (girl fingers + might be from her nasty "friends")
Tsuruko? (no nail polish unlike Anaru + might be from a Yukiatsu admirer jealous of her always being with him? lol)
or Jintan? (though it's very unlikely)