"We recognise many people have been affected by his passing and the school is readily available to provide support and guidance now, and in the coming weeks and months."
The Headmaster said the school is "devastated" by Hartner's death, and support is being offered to students.
O'Connor said students can directly communicate with him, the leadership team or Student Services, who have been working with students across schools over the past few days.
"Josh was a popular young man during his four years at Auckland Grammar; a talented design student who embraced life as a proud Grammar boy."
Hartner was described by the school as a promising rugby player. He also played social basketball and senior Saturday morning cricket "with his good mates".
"As a talented tennis player, he represented the School in a number of Premier tennis team fixtures," O'Connor's newsletter said.
"Most importantly, he was a loved son, brother and valued friend. Along with others we are helping support Josh's family and his friends across a number of Auckland secondary schools and feel the heartbreak with them."
The driver is assisting police with their inquiries. He underwent alcohol and drug testing after police arrived at the scene — a standard procedure after a crash.
Police could not comment on the results of that testing.
The serious crash unit is investigating to establish the circumstances of the tragedy.
Its investigation could take some time as its members speak to witnesses and complete various reports on the incident, vehicle, road situation and environment.
"It was a bunch of 17-year-olds having a good time and just gone horribly wrong," said a source.
Hartner's family have not yet commented on his death and have asked for privacy.
"We are ensuring that Joshua's family is supported as they grieve the loss of a dearly loved son and family member," said a police spokesperson.
Fifteen people have died during the holiday road toll period, which began on Christmas Eve and finishes at 6am on January 5.