Allow me to Speak! It is forbidden for the culprit to be anyone not mentioned in the early part of the story.
Allow me to Speak! It is forbidden for the detective to be the culprit.
Therefore, the murderer is either:
--Tatsuya
--Haruka
--"The Vice Prez"
--"The Queen"
Based on what I've seen so far, I'm 66% leaning towards the VP, and 33% leaning towards Tatsuya, but I suspect Tatsuya will be killed in the next few chapters.