talentless nana!

How come Yuuka didn't tell the class that Nana is the killer? Is she more concerned of her secret than the killer at large?

from the looks of it,she doesn't have enough proof to prove except for nana's word of mouth.
that and her horrible attitude of playing with her victims. D:

She has the murder weapon (needles) which is enough proof for Kyouya to confirm his suspicions. Then again she lives a lie to the others like Nana so she's not obligated to tell them.

And I didn't expect another twist in the end: Yuuka is a killer stalker xd

how did nana know that much detail about shinji's date...damn, that's some impressive detective deduction. XD

Maybe after noticing the loose necklace + protecting her back pocket she questions the validity of her story.

Also she has all the time to think about it while waiting for Yuuka to return.