When Sam McDougall regained consciousness, he and David Fellows, 19, and Luke Fisher, 17, rushed to the help of those lying around the wreck.
"Basically it was whoever needed to get to hospital, to get them in the car as quickly as possible," said the 18-year-old, whose arm remains in a sling after injuring his shoulder.
Luke used Mr Mmata's cellphone to contact the rest of the group, who were travelling ahead.
As other vehicles stopped, including taxis, injured passengers who weren't responding were put in the first cars to hospital.
"I think even now I still haven't grasped how big it actually is, and how much of an impact it's going to have," said Sam, who admitted he was acting on instincts.
Caitlin and Mr Mmata died at the scene, Mrs Johnston died en route to hospital and Dr Johnston died after surgery.
Teacher Jan Dean, who was left soaking, muddy and "completely out of it", guessed Sam's voice was the first she heard after the accident.
She and teacher Philip Russell, who was unconscious in the front seat after smashing his head against the windscreen, were the last two to remain in the van.
"I crawled up through the van and he was conscious and responsive, so I knew he was all right," she said.
"I could hear all sorts of things outside the van but I couldn't get out because it was on its side ... because I had broken my collar bone, I couldn't lift myself out."
She credited pupils Anna Boggiss, Joy Fisher and Esther Goddard for their support in hospital, as well as parent Kerri Tilby-Price, who had joined the trip as an observer but then found herself in charge.
The survivors recounted the generosity of Kenyan locals, including Calvine O'Minde, who had helped Mr Mmata, his best friend, run the Ark Quest School in Ma'hanga. Mr O'Minde stayed with them in hospital without sleep for five days before his exhaustion hospitalised him too.
Despite the tragedy, all of the group agreed they would happily return to Kenya, Mrs Dean vowing to head back "as soon as I'm healed".
"It's just an amazing place that has touched my heart," Sam said. The group had also been overwhelmed by the support from schoolmates.
Ten survivors are now home, with four still in hospital in Nairobi.
Funeral services will be held at the college for Caitlin today, and for the Johnstons on Saturday. Donations made to a Kenya families account total $27,000, and $11,000 has been given for the Johnston family.
