A teacher at the fictional Wittering College in north London, Robert Anderson (Schofield), is hit in the face by a pupil and forced to take three months' leave to avoid being sued by the parents of the child for giving the pupil an F grade, which is against school policy.
Anderson is deeply affected by the incident and upon his return to teaching, he is an alcoholic, emotionally disturbed and separated from his wife, Helen (Aubrey). Their daughter, Kate (Bennett), lives with her mother Helen and has classes with her father, but does not respect him. The headmistress, Sarah Balham (Gemmell), loathes Anderson and clearly wants to get rid of him, but the National Union of Teachers does not allow her to fire him. She indirectly accuses him of bad teaching and bringing alcohol into school.
When Anderson reads about violence in another school, he sends a memo advising all employees at the school about the high number of attacks on teachers and auxiliary staff annually. As a result, he is considered paranoid and delusional by everybody at the school.
One day, after hours, Anderson is overseeing detention, in which he has placed his daughter. They argue over her use of a mobile telephone during the detention session – he slaps her in the face and immediately regrets it.
Soon after, the protagonist notes some strange movements outside the school and discovers that the telephone lines are down. As he stands by a closed window, a milkshake is thrown at him from outside and the message 'U R Dead' appears written in the milkshake. He advises security guard James (Finlay Robertson), who seems uninterested.
Meanwhile, another security guard Brian, (Jamie Kenna) has been murdered by hoodies who locked him in a wheelie bin and set fire to it.
Looking for his daughter, Anderson visits the school library but, after he leaves, the librarian is confronted by two hooded characters, who murder her.
Anderson finds his daughter smoking in the toilets with her boyfriend, Jake Eaves (Max Fowler) and headmistress Balham instructs them all to go home. Kate tells Balham that her father slapped her and Balham asks her to make a statement. Meanwhile, PE teacher Nicky (McKee) passes through the gymnasium and into the changing rooms, where she is attacked by four teenagers wearing hoodies and carrying crowbars.
Having left the building, Anderson goes to his car, but finds the dead security guard's torch on the ground in the car park. Meanwhile, Kate is writing her statement in the staff room. Balham realizes she can finally sack Anderson and, unable to use the telephone, calls security guard James to ensure that the teacher does not re-enter the school, and Helen, to inform her of the assault.
Balham subsequently finds a body and escapes from two attackers herself, only for them to kill her before she can call the police. Anderson then discovers her corpse, her face badly disfigured. He tries to call the police, but when an attacker enters the room he hides, abandoning the phone, which the attacker crushes.
Leaving the building, Kate is immediately chased back in by one of the attackers. Her boyfriend Eaves, who was waiting outside, enters the building to look for her. She comes across a member of auxiliary staff Gary, (Tom Mannion), who is unaware of the situation in the school and goes after the attackers, but is electrocuted immediately after discovering Eaves, who is wrapped in barbed wire.
Anderson and James team up to find Kate, who has left a 'Help' note in view of a CCTV camera. They discover Nicky struggling along a corridor, but she has been badly mutilated. Finding the room in which Kate is hiding, they are confronted by a hoodie and in a moment of cowardice, James locks the door with Anderson inside.
Meanwhile, the police arrive and enter the school. A female police officer is pushed down a flight of concrete stairs by one of the hoodies and the other policeman is hit with a crowbar in the face. The attackers then find James, who tries to escape but is cornered by the youths.
Anderson finds Kate, but one of the hooded attackers catches the pair off guard, but his attack on Anderson is deflected and he stabs and wounds Kate. Anderson manages to overpower the attacker, grabs the knife and stabs him multiple times, then picks up his daughter and rushes out of the building. However, when they reach his car, they spot Helen's car in the car park and realize that she has arrived at the school.
Anderson has to decide in an instant whether to save his daughter's life by driving her to hospital, or to save Helen by re-entering the school. He puts Kate in the back seat of the car and drives her to the hospital, leaving Helen to walk around the school, unaware of her potential fate.David Schofield as Robert Anderson, English teacher
Eliza Bennett as Kate Anderson, his daughter
Ruth Gemmell as Sarah Balham, head teacher
Juliet Aubrey as Helen Anderson, Robert's estranged wife
Emma Cleasby as Lucy, school librarian
Finlay Robertson as James, school security officer
Roxanne McKee as Nicky Wright, PE teacher
Tom Mannion as Gary, school caretaker
Max Fowler as Jake Eaves, Kate's boyfriend
Jamie Kenna as Brian Stanton, school security officer
Ian Cullen as Maths Teacher