He is understood to have gone to his home nearby to change clothes after school was let out early, and had tried to catch the first bus to go shopping in New Lynn.
But its driver told him it was not going there, and he needed to catch the other bus instead.
A two-tonne goods truck travelling west along Terry St after rounding a corner from Boundary Rd then passed between the two stationary buses, hitting the young man and dragging him up to 15 metres downhill while braking.
"When he got hit he got dragged," said 17-year-old Laura Olbrich, who watched in horror from the first bus stop as her school friend disappeared under the truck.
"It [happened] really fast - I saw the truck coming and he was running across the road."
The victim's older brother said last night that his family was still too shocked to talk about the accident, although he suggested the location of the two bus stops may have created a blind spot.
Another school friend said he understood the victim, whose Year 12 courses included science subjects, had migrated with his family from Malaysia as a young child.
Barbara Donaldson, a neighbour who called emergency services to the accident scene while her husband Derek gave first aid by clamping a handkerchief to the young man's head to stem heavy bleeding, said his father had asked the couple yesterday for their prayers.
Mrs Donaldson, who heard a bang as the truck hit the young man, said earlier she did not believe there was anything its driver could have done to avoid the accident.
Her husband said the boy's body had ended up completely under the truck, with only his head clear of the front of it, but he was able to move his legs to assist ambulance officers once they arrived.
Lynfield College principal Steve Bovaird said he had not heard that his student was back on the critical list, and did not know all the facts of the accident, but a dean had spoken to the family and provided details for relatives to contact the school when they were ready.
He said he emailed all his staff yesterday, instructing them to remind their students about road safety.
"Young people do need reminding sometimes," Mr Bovaird said.
Another Lynfield College student, a boy aged 14, was hit by a car just two streets away at a pedestrian crossing at the top of White Swan Rd only about 20 minutes after the Terry St accident.
It's understood he was not badly injured.
- Additional reporting: APNZ