The accident happened at the intersection of Carr and Frost roads, just outside Mt Roskill Primary School and Mt Roskill Grammar.
O'Reilly said the "great little boy" was in pain with a broken leg but would be fine.
"I was talking to him about getting back to play cricket and he was able to smile."
Police are interviewing the driver and a Mt Roskill Grammar School student who witnessed the accident.
O'Reilly said in the three years he has been principal at the school he hasn't seen an accident outside the school like this.
"There is a lot of traffic but it moves very slowly. We don't have a history of accidents."
Police were called at 8.26am. The spokeswoman said the child had a broken leg and was treated at the scene.
St John and the Fire Service were in attendance as well.