The children ran to the field near the Bridge Marina carpark.
Ms Tong said she was putting out the fire with an extinguisher when a bus driver behind stopped and helped put the fire out with his extinguisher.
Police and fire services were quickly at the scene, but the fire was already out on their arrival.
Bethlehem College students were left temporarily stranded at the field, as they waited for a second bus or their parents to pick them up.
Ms Tong was helping calm the children and call their parents, despite being shaken by the incident.
Tauranga fire brigade senior station officer Neil Brown said the fire was relatively minor in the engine compartment.
When they arrived at the scene the fire was out and there were no injuries he said.
Go Bus regional operations director Darryl Bellamy said they would be carrying out an internal investigation to see how and why the fire started.