@smashman42
She gets sent to the hospital and starts displaying signs of it there, before they send her home. There the issue of them missing it is solved.
We don't know that she just drops dead instantly, the corpse appeared at 4 and she goes to the lunch at noon so she was dying on the roof...
I mean try to look at this from the perspective of someone who wasn't there and isn't reading a manga of the events. There's a picture of a guy with a knife, and another one at his feet, stabbed. There's another guy in the hospital stabbed. There is really only one conclusion you could draw from...