Okay, if I learned something from season one...........all of my researched info, is f*cking bullshit. XD
If you want to see those, and see how wrong I was, dig into my blog, it's not that far down.
My guess would either be Cecelia because honestly she seems likes stand-out girl and she seems a bit ordinary yet..........strange, something feels off, and that why I say that.
My other guess would be a staff member an in pic-tic-tac-u-larry, would be Mr.Pecs, why? Didn't anyone notice that he was gone for a bit before confronting Sister Benedine? Within the time we didn't see him he went and murdered poor Brendan.
All the other teachers are insane so they all have reasons.