The Arizona Republic reports that "The school did not contact police and did not provide counseling or an evaluate the boy to determine if he intended the drawing as a threat," which suggests to me that they did not really consider it to be a threat.
The boy's parents described the picture as a harmless doodle of a fake laser, which did not show blood, bullets, injuries, or target any human.
If the school really considered it a threat of an impending massacre, they should have treated it as one. Since they didn't, why is it even worth a suspension? Is there more to the context that we aren't being told, or are school administrators so irrational that they fear drawings of guns?
Are there any adult males who didn't draw guns along with cars, motorcycles, spaceships, monsters, aliens, and floor plans of secret hideouts when they were around 13?