She told him: “It is not your choice. You have to face the consequences of your actions, to hear the court’s sentence passed in public and it is important for those who loved your victims to see you sentenced." Prosper used a shotgun to kill his mother Juliana Falcon, 48, and siblings Kyle, 16, and Giselle, 13, and planned to shoot more than 30 pupils at a primary school.
The teenager, who had a "disturbing fixation with violence and obsessive brutality" was told he will serve at least 49 years in jail.
He avoided a whole-life order due to mental health conditions.
Mrs Cheema-Grubb said: "You intended to unleash disaster on the community of Luton.
"Your plans were intelligent, calculating and selfish. Your ambition was notoriety." She paid tribute to Prosper's family, saying "their deaths are almost certain to have saved the lives of many children", as it was a neighbour who called police after hearing a loud disturbance.
The judge's decision to force the convicted killer into the dock has added to pressure to ensure this happens in every murder trial.
It follows a series of high-profile criminals choosing to avoid their day of justice, to the distress of victims' families. Shadow Justice Secretary Robert Jenrick praised Mrs Cheema-Grubb. He said: "Brutal killers shouldn't be able to hide in their cells.
"These cowards should be forced to hear their sentencing, to face their victims' families and the devastation they have caused. The judge was right to force Prosper into court." Triple murderer Kyle Clifford, gangster Thomas Cashman and baby killer Lucy Letby are among those who have refused to attend sentencing after committing some of Britain's most heinous crimes.
Murderers have been able to hide in their cells after chaos engulfed plans to change the law.
The Tories under Rishi Sunak wanted to give judges the power to compel murderers to attend their sentencing hearings.
But the Criminal Justice Bill, which contained the measures, did not pass through Parliament before last July's general election.
Sir Keir Starmer's government has confirmed it plans to change the law to this effect.
Det Chief Insp Sam Khanna, who led the investigation, said Prosper's mother Juliana, brother Kyle and sister Giselle "were brutally murdered by someone they loved, who they should have been able to trust" at their flat in Luton early on September 13.
Their murders, which Prosper admitted, were part of a wider plot to storm a morning assembly at his former school - St Joseph's Catholic Primary - with a shotgun.
The court heard he managed to forge a gun licence and used it to buy the firearm and 100 cartridges the day before. Prosper had even chosen a black and yellow Kill Bill costume uniform for the atrocity and picked Friday 13 for the attack....