Sam said he had no idea how serious the crash had been and, having never been in such a situation, was acting on instincts.
Luke meanwhile took the cellphone from driver Christopher Mmata, who also died in the crash, and phoned the rest of the group, who were travelling ahead in a car.
The vehicle turned around and those in it, among them Anna Boggiss, 16, and Luke's sister Joy Fisher, 19, helped assist the others.
Sam said he was heartened to see taxi drivers volunteer their vehicles to help the injured to hospital.
Caitlin and Mr Mmata died at the scene, Mrs Johnston died en route to hospital, and her husband died after surgery.
The group told how they stayed strong together in hospital, and had been humbled by the support received from New Zealand.
"I'll admit I shed a tear," Sam said.
Ten survivors have now returned home, while four remain in hospital.
Earlier today, the survivors shared emotional reunions with classmates at a morning vigil at the college hall, where they shared stories about the trip, sang and hugged one another.
Funeral services will be held at the college for Caitlin tomorrow, and for the Johnstons on Saturday.
Donations made to a Kenya families account set up by the college stands at $27,000, and a further $11,000 had been donated to assist the Johnston family.
Account details could be found on the college website.