Both father and the son are evil. This seems to extend to the grandmother as well, and only the mom's being the tool around.
Later, the boy will make a confession to the whole school. This ruins the MC's plan but I think she'll find a loophole.
He had murdered his former teacher through arson and was covered up by his father I believe. He has committed another arson through proxy? He manipulated and blackmailed a girl into doing it.
He also threatened to harm the glasses girl's cat.
So far nothing has happened on MC's side yet. I'm guessing she gets proof and tells the boy that he hasn't confessed his other dark secret so the game will continue to move forward. This will piss off the boy and the father gets involved in this mess. I bet MC will convince the mom to help her out with the plan. I wouldn't surprised if the granny gets caught in a crossfire and becomes a collateral damage.