Aenigma

At St. Mary's College, a boarding school for girls located just outside Boston, Massachusetts, a lonely, gaunt, unattractive student, named Kathy (Mijlijana Zirojevic), becomes the butt of a cruel practical joke perpetrated by a gang of students and the hunky, but sadistic gym teacher, Fred Vernon (Riccardo Acerbi). Kathy is tricked into going on a date with Mr. Vernon, but when she overeagerly makes her move onto him in his parked car in the woods, the students appear in their cars to taunt her. Running away from several carloads of her tormentors, Kathy runs onto a busy street where she immediately gets hit by an oncoming car and is admitted to the local hospital in a coma. Kathy's strange mother, Mary, continues to gloom around the school in her capacity as a cleaner.Some time later, a new girl named Eva Gordon (Laura Naszinski), arrives at the school where she is shown around by the stern headmistress Miss Jones, and given Kathy's old room. Eva becomes acquainted with several of the perpetrators of the stunt. It is soon revealed (to the viewers only) that Eva is Kathy. Eva has somehow been processed by the spirit of the comatose Kathy whom begins using supernatural powers of telekinesis and magic to exact revenge against all those involved in the prank that put her in a coma. The first victim is Mr. Vernon, whom asks Eva on a date. That night while waiting for Eva to show up at the gym for their secret rendezvous, the narcissist Mr. Vernon is attacked and strangled by his own reflection when it jumps out of a mirror. The following morning, the police detective on the scene (director Lucio Fulci) concludes that Mr. Vernon died of a heart attack.The following night, Virginia Williams (Kathi Wise), one of the students involved in the prank against Kathy, is attacked in her bed and suffocated to death by an onslaught of snails that appear and then dissappear. Again the police do not have any explination for the mysterious death.Meanwhile, the neurologist in charge of Kathy's case, Dr. Robert Anderson (Jared Martin), notices that Kathy's vital signs increase in step with each of the mysterious deaths at the school. But he cannot make a connection to it. The next morning, Dr. Anderson is called to the school to examine Eva when she has a violent breathing fit during a class. During that same time, Kathy's respirator temporally breaks down which leads to Eva's respiratic attack, but nobody makes any connection to it. Afterwards, the strange Eva comes on strong to Dr. Anderson, leading him to ask her on a date where afterwards she seduces him.During a field trip to a local art gallery, Grace, another one of Kathy's tormenters, returns to the gallery when it closes to search for a valuable piece of jewelry that she left behind on the day visit. A marble statue comes to life and falls down on her, crushing her to death.Dr. Anderson starts to become more apprehensive about the intense Eva. One evening, he suffers a horrible nightmare where she kills him after sex. He's relieved when Eva's mother arrives at the school to take her home for some rest. Eva begins to write obsessive love notes to him, but he brushes them off. Soon, Anderson becomes involved with another girl, named Jenny Clark (Ulli Reinthaler), the only student who shows any remorse for the prank gone wrong against Kathy.Eva runs away from home and returns to the school after dark. Eva sets her eyes on Kim (Sophie D'Aulan), one of Kathy's cruelest tormenters. As Kim is preparing to meet her boyfriend Tim for a date, Eva makes Kim see horrific visions of Tim being decapitated at every turn. Kim suffers an anxiety attack and in attempting to flee from the gory visions, plummets out of a third floor window to her death. Tim arrives seconds later, looks out an open window, and sees Kim lying dead on the pavement below. Tim is decapitated for real when Eva commands the window blind to fall down on his neck. Eva quietly leaves the building, while Kathy's mother, Mary, watches from a distance.At the hospital, Jenny arrives for a late-night date with Dr. Anderson, when she gets lost in the gloomily lit building, thanks to Eva using Kathy's magic skills of mind bending. Jenny ends up in the morgue where she encounters Eva. The possessed girl, grabs a scalpel and threatens to kill Jenny for taking away her man, but Anderson arrives in time to intercept the slash of the scalpel, but gets badly slashed on his forearm. As Eva rises the scalpel to stab them both, she suddenly freezes, gasps for a few seconds, and falls to the floor dead. Upstairs in Kathy's hospital room, her mother has pulled out all her intravenous drips, removed the respirator and turned off the life support systems. Appalled at the supernatural malice of her venganced-crazed daughter, Mary has somehow figured out of Kathy's connection to Eva and has decided to put her daughter to rest. Kathy's soul rises out of her body as a bright blue light, and disappears into the night sky.

Question: How was Grace killed the the plot.