A teenager who murdered Leeds school teacher Ann Maguire in front of other pupils has been handed a minimum of 20 years' detention.

The 16-year-old, who cannot be named, killed the Spanish teacher at Corpus Christi College in April.

He had previously "accepted responsibility for unlawful killing" but issued a formal guilty plea to the murder charge at Leeds Crown Court.

The court heard that the boy had planned to kill two other teachers, including one who was pregnant, but had "harboured a deep-seated and irrational hatred" for Mrs Maguire and "wanted her dead".

He had sent Facebook messages speaking of "brutally killing" her and told a friend on the social networking site that she "deserves more than death, more than pain, torture and more than anything that we can understand."

But when he told schoolmates that he was going to attack Mrs Maguire with a knife, the court heard "many did not take him seriously".

He had even showed some of the other pupils the knife on the day of the murder, telling them what he planned to do but said he would "kill them" if they told anyone else.

Prosecutor Paul Greaney QC said the teenager stabbed Mrs Maguire seven times in the neck and back as she leaned over a desk to look at another pupil's work. 

When she tried to run away, he chased her.

Another teacher saw Mrs Maguire running down a corridor, saying: "He's stabbed me in the neck", with the boy pursuing her. The female teacher pushed her colleague into a workroom and then blocked the door to prevent the boy getting in.

The teacher said that, as Mrs Maguire lay injured, she spoke about her children and the fact that she was loved. A paramedic who tried to save her said later that the stab wounds were the worst he had ever seen.

Prosecutors said that the boy's parents are "decent and responsible" and are at a loss to understand what has happened.