I’m not too worried about the blood as it could have come from anyone. I doubt Ayumi and Yandere girl were the only two in that part of the building.
I said this before but I really don’t like the Yuu and Nao pairing yet we keep getting “you two would be perfect” shoved down our throats. If Ayumi really did die (which I doubt) I’m really looking forward to see how Yuu changes.