Warning This Section Does Contain Spoilers
There can only be one suspect eliminated this episode based off of the evidence. Aura McBride was cleaning at the school until 9:45pm and Arthur alibis her for being there. She is completely cooperative with police and calls them as soon as she finds Charlie's body. There is no apparent motive for her to kill her cousin.
We should be able to alibi Arthur Hughes as well based off the same principle that Aura can alibi him from them cleaning the school but that's not the case based on the episode recaps. Could this be a clue?