A senior coroner for Liverpool and the Wirral said CCTV footage from inside the coach shows the bus driver, Stephen Shrimpton, 40, suffering an “event” which saw him slump to his left while at the wheel.
The coach left the carriageway and went up an embankment before ending up on its side on September 29 at around 8am in the morning.
It was carrying children from West Kirby Grammar School and Calday Grange Grammar School when it hit the reservation between Junction 5 and Junction 4.
The coroner said Jessica Baker, 15, suffered “instantaneous” death and she was a passenger along with around 50 other youngsters on the bus.
Andre Rebello, the senior coroner said it was “miraculous” no one else was killed as he gave brief details of the incident during the opening and adjourning of the inquests into the deaths of Jessica and Mr Shrimpton at Liverpool Coroner’s Court.
He stated more “clarity” was needed from the government about the wearing of seat belts on coaches and buses.
Mr Rebello said though further medical examination results are to come, the most likely cause of death for Jessica will remain a severe head injury.
Jessica’s family have since released a tribute a statement released through Merseyside Police. In it, they said: “Jessica was a warm-hearted, wonderful daughter, granddaughter and niece, devoted sister and loyal friend.” Mr Shrimpton’s family described him as a “loving husband and father”.
At the coroner’s court, Mr Rebello added: “At approximately 8.03am the road traffic incident occurred and the coach, travelling north bound, collided with the embankment on the nearside and the coach came down on its side, causing fatal injury, life-changing injury and serious injury and lots of minor injuries.”
The inquest hearing was told Mr Shrimpton was pronounced dead at the scene at 8.50am and formally identified by his wife Tania Shrimpton.
Jessica was also pronounced dead at the roadside at 9.01am, and identified by her father, a nurse, Mr Shaun Baker.
After the crash, four other children were taken to hospital including a 14-year-old boy whose injuries are said to be “life-changing”, according to Merseyside Police.
A full inquest will be heard on March 20 next year.