Go to the police first, then school board, then media as last resort.
I'd first call the cops and get a police report on file. Then, I'd call the county school board superintendent and report the principal. If they don't do anything, sue from the top down. A firing of the principal would be sufficient for me. The proper policies and protocol's are already in place. The problem is with a failure to execute, which is the principal's job.
I understand you're pissed, but the media is like setting off a biological weapon. You really have no control over the fall out, and your pro-gun rights will likely be affected, especially since it's a small town. However, if you want to turn your idyllic town into a mini-California, by all means, report criminal gun violations every chance you get.
We always complain about how anti-gun the media is. However, many here are now taking the opportunity to feed the monster. Do you really think the media will spin this story in a pro-gun manner? What exactly do you think the headline of the story will be?
The headline will certainly be derogatory towards guns. The principal may even come out looking like a hero somehow...seriously. The plight of your girlfriend will be lost underneath the sensationalism about the "fully automatic AK-47 assualt rifle" that the kid brought to school.