Cornick stabbed Mrs Maguire, 61, on 28 April this year as she taught a class at Corpus Christi Catholic college, in Leeds, and was jailed for a minimum of 20 years after pleading guilty to murder at Leeds crown court.
Mr Justice Coulston warned that he may never be released, saying he had shown a “total and chilling lack of remorse”.
Prosecutor Paul Greaney QC told the court: “It is important that we should record that it is clear from the evidence that the parents… are decent people and responsible parents.
“They are at a loss to understand how and why their son has turned out as he has, and they have cooperated fully with the police and with the prosecution.
“It follows that this is not one of those cases in which a defendant’s actions may find a degree of explanation in his family circumstances.
“On the contrary, (the boy’s) family life was marked by love and support.”
Mr Greaney said there had been nothing to indicate to the boy’s parents or teachers a risk of “homicidal violence”.
But, he said, pupils had noticed disturbing aspects to his personality. The boy told other children that he hated Mrs Maguire and wanted her dead.
The prosecutor said that late on the night of Christmas eve 2013 and into the early hours of Christmas day, “the defendant exchanged messages with a friend of Facebook.
“In those messages he spoke of ‘brutally killing’ Mrs Maguire and spending the rest of his life in jail so as not to have to worry about life or money.”
And on the morning of the murder, the boy had told pupils he was going to attack Mrs Maguire, showing some of them the knives he had with him.
There was no expression on the boy’s face as he stabbed the teacher, the prosecutor said.
The defendant, wearing a grey suit and tie, looked straight ahead and showed no emotion as he admitted murdering Mrs Maguire.
The teenager’s parents sat at the back of the dock, listening intently to details of their son’s crime.
The Maguire family sat in an area of the courtroom normally reserved for court officials, while the large public gallery was packed with members of the press.