For the curious, I looked it up. The reason he had access to a 6th grader to shoot in a High School is that it and the Middle School are really one big building in this case - so while it was a High School, it was also a Middle School.
Principals avoid taking actions that will put their campus in a negative light. They will call the police only when the situation cannot be hidden from the public such as a gun, assault with a weapon, or criminal trespassing.
Teachers can only enforce the policies that the admin are willing to enforce. They're also discouraged from pressing charges under that of losing their job.