Dr Stephen Moore, head of Modes Study Centre in Oxford, said that, at first, Petrolini, 19, evaded the question by rambling on about the SAS and proving himself before he could join the unit.
'I asked him for the umpteenth time, had he committed murder, and to my surprise he said yes,' said Dr Moore.
Dr Moore said that when he telephoned Petrolini's father in Granton-on-Spey, Highland, he said his son had confessed to him on the phone the night before.
The court heard that the principal intervened after Petrolini had made a similar confession to Jonathan Rowse, a flatmate and fellow pupil. After hearing the confession, Mr Rowse barricaded himself inside his bedroom.
Police said that when they arrived at the school, Petrolini claimed Richard Elsey, 19, a fellow pupil, was an SAS officer and had been with him when he committed the murder.
The prosecution alleges that the teenagers jumped into the car of Mohammed el- Sayed, 44, in Bayswater, west London, on 14 January and that Mr Elsey restrained him while Mr Petrolini slit his throat and stabbed him in the chest 12 times.
Mr Elsey of Beaconsfield, Buckinghamshire, and Petrolini, a former Gordonstoun pupil, deny murder. Petrolini has admitted manslaughter on the grounds of diminished responsibility. Mr Elsey says he thought Petrolini was only going to rob Mr el-Sayed.
Dr Moore said the teenagers were 'immature'. Petrolini had been reported out late at night 'all blacked up' and the boys had also been arrested for climbing a crane. The week before Petrolini's confession his behaviour had become strange.
The trial continues today.Reuse content