A 16-year-old boy has pleaded guilty to stabbing a teacher to death in front of a horrified class in Leeds.

The boy, who cannot be named, admitted murdering Ann Maguire, 61, in her classroom at Corpus Christi Catholic college on 28 April this year. Her family were in Leeds crown court to hear his plea.

The youngster was 15 and studying for his GCSEs at the time of the attack. His parents sat with him in the dock at Leeds crown court as he admitted the killing. The prosecution has argued that the boy should receive a minimum 12-year tariff because he "derived pleasure from the public nature of her killing”.

Maguire’s daughter Emma told the court, "Every morning I wake up hoping it’s all a bad dream. There’s a split second before reality sets in.” Maguire’s husband, Don, said: "There will be no closure. Balance will never return. There will be no level scales.”

The court heard that Maguire was the boy’s Spanish teacher. The boy was a model pupil in year 7, was "amicable, enthusiastic and conscientious” but changed after being diagnosed with diabetes. The diabetes had a major impact on his mood and personality. His mother noticed evidence of self-harming.

He was upset that his diabetes would prevent him from joining the army. He started to harbour a hatred for Ann Maguire, the court heard.

The boy told other pupils he had been carrying a knife and wanted Maguire dead: on his mobile police discovered many images of knives. On Christmas eve 2013 the boy told a friend on Facebook he wanted to "brutally kill” Maguire and spend the rest of his life in jail.

The boy told one friend he planned to kill two other teachers as well as Maguire — one was young and pregnant.

On the day of the murder the boy took knives to school and a bottle of Jack Daniel’s whisky in order to "celebrate afterwards”.

The attack took place at Maguire’s desk. The boy was in her Spanish lesson. He told a pupil of his plan and winked as he left to kill her.

He attacked Maguire from behind, the court heard. She was 5ft 2in: a foot shorter than her killer. He was expressionless as he stabbed her in the neck and back.

After stabbing Maguire and chasing her out of the classroom the boy calmly returned to his desk and, according to witnesses, said: "Pity she didn’t die.”

The court then heard harrowing video testimony from pupils who witnessed the attack. "I was in absolute panic,” said one classmate.

Susan Francis, a colleague of Maguire’s, heard screaming. She rushed into the corridor and was confronted by pupils running, screaming. Maguire was holding her neck and said "he’s stabbed me in the neck”. The boy followed her. The teacher pushed Maguire into another classroom. The other teacher held her foot against the door to stop the boy entering. She could see his "emotionless” face through the door window. Francis sat with Maguire comforting her and telling her she was loved.

The boy returned to his seat after the murder and said "good times”, according to witnesses. He spoke of an adrenalin rush. One pupil said it appeared that he was pleased with what he had done.

Two teachers entered the room and the boy put his hands in the air as if in surrender. He told them of "other naughty stuff” in his bag. When police arrived they were struck by his "calmness and air of normality”. He chatted with officers about their hobbies, the court heard.

The boy told a psychiatrist he did not regret his actions and was pleased to have killed Maguire but "it didn’t live up to” his expectations.