The van drove off and William ran alongside it, but was unable to keep up.
He fell over and was dragged for about 30-40 metres until he broke free from his bag and was left at the side of the road.
William suffered minor scrapes and bruises and was airlifted to Taranaki Base Hospital.
Mr Russ said it was very fortunate William had come free from his bag.
"Otherwise the other result wouldn't have been a good one,'' he said.
"If he hadn't been able to get himself free from that bag, then it doesn't bear to think about what the other one would have been, because the driver had no idea that he was still attached.''
Police had spoken to the driver, who was contracted to drive the school-owned minivan.
They were still considering whether to lay charges.
Mr Russ reminded school bus drivers to be more vigilant about children around their buses.
"It only takes a moment's inattention, really.''
Taranaki rescue helicopter chief crewman Jayden Strickland said the boy had suffered minor to moderate gravel rash across his body, with most of the injuries on his buttocks.
"It would be sore, but he was a brave little man. No crying at all, and fully conscious and talking to us about the whole situation.
"He was just dragged by his school bag in the bus, which had a manual closing door. They rely on the children closing the door by themselves,'' he told APNZ.
William was still in hospital mid-afternoon - largely because of the length of time needed to clean up the large number of grazes he suffered.
Police said he was likely to be discharged from hospital today.
Huiakama School principal Vivienne Wallace said the rest of the children at the 10-pupil school - including William's little sister - did not see what happened.
They were already off the bus when William's bag got caught in the bus door.
A Fire Service education officer who was invited to the school regularly to talk to the children about safety made a special visit today to explain what had happened and how it would be dealt with.
Ms Wallace said a number of people from around the area popped in during the day to ask if everything was alright and to offer help if needed.
"We have a very supportive community.''
However, they did not stay when they learned everything was alright, leaving the principal and students to get on with their work.
She said she had spoken to the driver who was "very shaken''. He did not do the after-school run today.
The pupils were picked up by family members who were driving past on their way home from Christmas shopping or other errands.
Ms Wallace said the morning's incident wasn't portrayed as a big drama.
"Remember, this is a farming community. People just carry on.''
The Ministry of Education said the minibus was not ministry-funded or resourced.
The ministry has been in touch with the school, which has said it would comply with any recommendations made by police.
