So i can only assume something happened with the Brother that is motivating her to do these actions... the question is what and why. The people shes targeting don't even seem to know her brother.
@Ghostdevil My theory would be that maybe the girl who killed herself was the brother's girlfriend and thus he wanted revenge against the people responsible (maybe he also hurt himself because of his girlfriend's death and thats why his sister wants revenge). So maybe now they transfered to this School to take revenge on Gyeong Hwa. And the teachers mentioned that they also cause some kinda Trouble at their previous School, so maybe there they already took revenge on Lee Jae Yeong.