UPDATE (1/9/12): According to that article:
According to a report by KPNX, police and school officials said the weapon was a .22-caliber pistol.
Parent Mike Place told KSAZ TV he's grateful nobody was hurt.
"I think it's a miracle for a handgun to be fired in close proximity on a bus and not have any of those students grazed, much less hit," he told the station, "It's a miracle and a blessing thank God none of them were hurt."
The station reports that the boy acquired the weapon from a closet at home and had the gun with him all day at school.
Every gun in the hands of a child must first pass through the hands of an adult.