Justine Fielding, Tuppence Middleton, the head girl at Fairview High, is escorted out by the police as her classmates watch.
Five days earlier, Justine stands to read the eulogy at the funeral of Darren Mullet [Calvin A. Dean], an unpopular, asthma-suffering student whose death exposes the cruelty that festers within the school. Mullet’s equally unpopular friend, Jason Banks [Olly Alexander], is ripped from the church by a sadistic P.E. teacher after he calls Justine a hypocrite because he hardly knew him. Later, Justine agrees to attend a party with Alex [Dimitri Leonidas], organized by his popular circle—Bradley White [Alex Pettyfer], Natasha “Tasha” Cummings [April Pearson], Khalillah [Larissa Wilson], Sophie [Georgia King], and Marcus [Tom Hopper].
At the party, the DJ Jez [Ben Lloyd-Hughes] is egged on—by Tasha—into rapping unflatteringly about Justine, and is tossed out. Bradley’s crew launches a barrage of insulting texts from Mullet’s number to the party-goers, including Marcus and Sophie. When Justine and Alex slip away to a bedroom to kiss, Bradley interrupts with a prank: a clown costume and a pretend chainsaw. The in-crowd later toasts Mullet, and Justine confesses she never really knew him. Jez later goes to the cemetery and urinates on Mullet’s grave, only to be stabbed with a wooden crucifix by the ghost.
Back at school, Bradley threatens Nasser [James Floyd], the leader of the emo clique, over removing a website he helped create about Mullet. Justine abandons her old friends Helena [Mary Nighy] and Emily [Ruby Bentall] to chase the popular crowd. A teddy bear—stolen from Mullet’s grave—appears in Justine’s locker, and she assumes it’s from Alex, agreeing to a second date. Jason tells her that Mullet loved her and hands her his suicide note. In the recording studio, Mullet’s ghost compels Nasser to listen to blaring music through his headphones, leaving him permanently deaf.
Bradley and Marcus attack Jason, believing he sent the messages, but another message arrives that Jason could not have sent. The P.E. teacher arrives, dismissing the bullies and detaining Jason. Justine confronts Jason; he denies forging the note and confesses that Mullet killed himself because of the bullying, including the cruel website and texts from the popular students. The girls assault Justine’s former friends Helena and Emily; Tasha shatters her own phone after being accused of sending the messages. After swimming practice, Sophie urges Justine to sleep with Alex that night, but she returns to retrieve her watch and is drowned by Mullet.
Mullet’s ghost brings flowers for Justine, but after witnessing her kiss with Alex, he tears the badge from her uniform and rearranges her fridge magnets, calling her a “dirty slut.” During football training, Marcus is driven to hysteria by the apparition and is nearly blinded by a towel-whip, then impaled on an iron fence by the ghost. The gang later quarrels at Bradley’s house; Tasha fights Justine and throws her into the swimming pool.
Justine learns from Alex’s website that she could have helped Mullet and discovers that Alex led the bullying. The relationship ends as she admits she did not help Mullet when he needed it. Bradley, broken by grief, has sex with Tasha 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.
The next day, Justine tells Jason that she was responsible for Mullet’s death; he accepts some blame, saying he was afraid of being bullied, too. After Justine leaves the Art Room, Mullet stabs Jason with a screwdriver, killing him. Justine pleads with her old friends Helena and Emily to take her back, but they refuse, choosing to stay with Alex and the others.
Khalillah tells Justine she received a text from Tasha to meet in the Art Room. There, Mullet places a plastic bag over Khalilah’s head and then cuts off her hands with a guillotine. Justine and Alex arrive to find Jason and Khalillah dead and are attacked by Mullet. They stab him with a screwdriver and move to the common room, where Mullet nails Alex’s hand to the floor. Justine stops him, but he regains strength thanks to another inhaler. Mullet then impales Alex in the throat with the screwdriver before disappearing. Police arrive, having found Justine’s badge near the bodies of Bradley and Tasha, and she is led away in a police vehicle, arrested for the murders that Mullet had committed.
