Plot
Head girl Justine Fielding (Tuppence Middleton) is escorted out of Fairview High School by the police, as other pupils look on.
Five days earlier, Justine is reading the eulogy at the funeral of unpopular, asthmatic student Darren Mullet (Calvin Dean). Mullet's equally unpopular friend, Jason Banks (Olly Alexander), is (literally) thrown out of the church by the sadistic P.E. teacher after calling him a hypocrite because he did not really know him. Later, Justine agrees to go to a party with Alex (Dimitri Leonidas), organised by his popular friends, Bradley (Alex Pettyfer), Tasha (April Pearson), Khalillah (Larissa Wilson), Sophie (Georgia King) and Marcus (Tom Hopper).
When Justine arrives at the party, the DJ, Jez (Ben Lloyd-Hughes), egged on by Tasha, raps unflatteringly about her, before being thrown out of the party by Bradley. Bradley, Tasha, Khalillah, Marcus and Sophie all receive insulting text messages from Mullet's number. Later, Alex and Justine go to a bedroom and make out, only to be pranked by Bradley, wearing a clown costume and pretending to attack them with a chainsaw. Later, the 'in-crowd' toast Mullet. Justine admits that she did not know who he was. Jez goes to the cemetery and urinates on Mullet's grave. He is stabbed with a wooden crucifix by Mullet's ghost.
At school, Bradley threatens Nasser (James Floyd), the leader of the school's emo clique, about removing a website he had put up about Mullet. Justine drops her old friends Helena and Emily to hang out with the popular crowd. She finds a teddy bear (stolen from Mullet's grave) in her locker and, assuming it is from Alex, agrees to go on another date with him. Jason tells her that Mullet was in love with her and hands her his suicide note. In the school's recording studio, Mullet's ghost forces Nasser to listen to music at top volume through his headphones, rendering him permanently deaf.
Bradley and Marcus assault Jason, who they believe to be sending the text messages, but they are interrupted by another message, which Jason could not have sent. The P.E. teacher arrives, lets the bullies go and gives Jason detention. Later, Justine confronts Jason. He denies forging the suicide note, and tells her that Mullet killed himself because of bullying by the popular students, including vicious texts and a website that Bradley's gang created about him. Alex's friends tell her she cannot hand the note to the police, because it accuses Justine of hating him, thus supposedly implicating her in his death. The girls attack Justine's former friend Helena in the toilets; Tasha smashes her phone after accusing her of sending the messages. After swimming practice, Sophie tells Justine that she should sleep with Alex that night, but goes back into the swimming pool to retrieve her watch. She is attacked and drowned by Mullet's ghost.
Mullet's ghost brings flowers for Justine, but, after seeing her have sex with Alex, tears the badge from her uniform, and re-arranges her fridge magnets, calling her a "dirty slut". During football training Marcus becomes hysterical after seeing Mullet's ghost, and the P.E. teacher sends him off for a shower. Mullet's ghost then whips him with a towel, almost popping one of his eyes out its socket. He fights Mullet off with a cricket bat and gets outside, but is impaled through the skull upon an iron fence by the ghost. Later on, the gang have an argument at Bradley's house about who killed their friends, ending in Tasha fighting Justine and throwing her into the swimming pool.
Justine demands that Alex shows her the website, where she tearfully witnesses the in-crowd bullying Mullet. Justine realises that she ignored Mullet's pleas for help because he interrupted a conversation about her going to Oxford University. She tells Alex that their relationship is over.
After trying to dig up Mullet at the cemetery, Bradley breaks down and cries over the death of his friends. Tasha consoles him and they have sex in the car, but Mullet drags him out and rips off his penis, causing him to bleed to death. Tasha escapes and runs into an open grave, where Mullet decapitates her with a shovel.
Next day, Justine tells Jason, who is in the Art Room, that she was responsible for Mullet's death. He tells her that it was his fault, because he was scared of being bullied, so he told Bradley that Mullet fancied her. After she leaves the room, Mullet jams two pencils up Jason's nose and slams his head against the table, killing him. Justine vainly begs her old friends Helena and Emily to take her back, telling them that her new friends, including Alex, were all horrible.
Khalillah tells Justine that she received a text message from Tasha (who she does not know is dead) to meet in the Art Room. There Mullet places a plastic bag over Khalilah's head, before cutting off her hands with the guillotine. Justine and Alex arrive at the Art Room and find the bodies of Jason and Khalillah, and are attacked by Mullet. They stab him with a screwdriver and head to the common room, where Mullet nails Alex's hand to the floor with the screwdriver. Justine stops him, but he begins to choke her to death. She takes Mullet's inhaler and throws it across the room, telling Alex to break it. He does, and the ghost begins to die. Justine tells Alex to leave, but he finds Mullet's other inhaler, which Alex had hidden from Mullet when he was alive. Mullet then uses it, restoring his strength, and stabs Alex in the throat with the screwdriver before disappearing. The police arrive, having found Justine's badge near the bodies of Bradley and Tasha. She is led out into the police vehicle.
In a mid-credit sequence, the bullying gym teacher gives a speech to his students about how he was angry that Justine had killed his best players and ask why she did not kill them instead he sends the team away only to see Darren sitting, ready to kill him.
