Robbie Gemmell, 17, originally told police officers that he was a front seat passenger in a Peugeot 206 that crashed on a single track road in Tyninghame, East Lothian, in November last year.
The detail emerged after Gemmell, from Dunbar, pleaded guilty at Edinburgh Sheriff Court to causing the deaths of David Armstrong, 15, Jenna Barbour, 18, and Joshua Stewart, 16, by driving a car without due care and attention.
The victims were all pupils at Dunbar Grammar School. Members of Gemmell's family and relatives of the deceased wept as they heard about the incident from the public benches.
The court heard how the quartet were travelling in a Peugeot 206 when it struck a large wall support on an unclassified stretch of road known as Limetree near Dunbar.
The court heard how accident investigators reckoned the Peugeot was travelling at a minimum of 50 to 54 miles per hour in the moments before the collision.
David Armstrong was thrown from the car on impact and emergency service workers found his body 15 metres from the vehicle.
The court heard how Gemmell originally told officers that Miss Barbour - who was due to go travelling in New Zealand in January this year - had been driving at the time of the fatal collision.
Prosecution lawyer Graeme Jessop said that Gemmell - who was in hospital recovering from his injuries at the time of his confession - told his father that he had been the driver.
Mr Jessop added: "The accused became upset and started to cry. His father, Mark Gemmell, asked him if he had something to say and he said 'Dad, I was driving the car'.
"The accused and his parents were all in tears and his father told him to tell them everything.
"He proceeded to do so. Robbie Gemmell told his father that he drove down the road, he didn't specify how far or for how long but that he thought that he was doing around 45mph.
"He remembers being on the road but Jenna having to grab the wheel to straighten them up.
"Robbie remembers pulling the steering wheel back the opposite direction from Jenna and then they suddenly struck a wall.
"When he had finished his account, his father said that he would speak to the police about it and that Robbie Gemmell would have to tell them everything he had just told them."
After hearing the circumstances, Sheriff Gordon Liddle extended his sympathy to everybody involved.
Deferring sentence for reports, Sheriff Liddle added: "I do not want to deal with this case but it is my duty."
Gemmell, who has no previous convictions, wept uncontrollably as Mr Jessop read out the circumstances surrounding the case.
Mr Jessop told the court that the car involved belonged to Miss Barbour. At some point in the evening, Gemmell took over driving from her.
The court heard that at 8.15pm, Gemmell made a 999 call to ambulance control.
Mr Jessop said: "Due to the caller being in a distressed and disorientated state, it took between 15 and 20 minutes to establish the caller's location. This caused difficulties and delay in the emergency services tracing the whereabouts of the collision."
The court heard that the police arrived at the collision scene at 8.55pm.
Mr Jessop added: "About 20.55 hours on the same date, police officers and paramedics arrived at Limetree Walk, Tyninghame, where they observed the accused waving at them.
"They immediately rendered assistance to the accused and checked on the conditions of the passengers.
"It was apparent that the three passengers had all succumbed to their injuries."
Defence solicitor advocate John Scott QC described the incident as an unspeakable tragedy.
Gemmell will be sentenced on September 19 at Edinburgh Sheriff Court. Inspector Richard Latto, of the local Divisional Road Policing Unit and the senior investigating officer in the case, said: "This tragic incident reinforces the extensive risks of an inexperienced person not legally old enough to possess a driving licence, getting behind the wheel of a motor vehicle."