My first guess would be Io. Her obsession with Akira and how badly she took his words stand out to me. I can see her overdosing, either on purpose or by accident. Could also see a pill overdose as something the girls would try to 'fix'. Although considering in the text messages they're all starting to go insane, someone could have been brutally murdered and they're trying to shove her organs back inside. But Io feels like the most 'realistic' option to me, without any further detail about it.
And while I'm mindlessly speculating, I believe the way Otoha will eventually sleep with Sensei will be out of spite from getting continuously rejected by Niki. I think she even started to tell Niki that Akira is sleeping with other girls before Niki interrupted her and got angry. So with the darker way her character is going, I can definitely see her getting to a point where she gets into a "if I can't have you, I'll have your lover" kind of mindset.