I'm not saying she should have criminal charges. I'm saying she is lucky she doesn't. Back when I was in school you were allowed to have your shotgun or rifle in your vehicle if you were heading to school from hunting or heading out right after school. The issue I have is people wanting to make special rules because the girl is on the honor roll. Did she have intent to harm anyone? No, but that doesn't change the fact that she could be facing SERIOUS charges if the school system and police felt she needed to face those charges. She would've been better off if she could've gotten a family member to come with the keys and take the vehicle off school property. If it would've been me, I wouldn't have called the office to tell them I had a rifle in my trunk. My call would've been to a family member that had a spare key, which at least 3 did when I was in high school.
In my opinion, she should not be punished because there was no indication of intent to break the rules or harm another person. She is a teenager; she forgot.
The fact that she could face serious criminal charges is because the laws are stupid and were passed by micro-managing buffoons practicing CYA in the extreme.
BTW, calling a family member might have gotten her expelled for using a cell phone in school (presumably to make a drug deal, don't you know). You can't use a phone in the office because you have no expectation of privacy and all student calls are likely monitored.
In the old days, when people in authority had common sense, made decisions, and took responsibility for those decisions, she would not have been punished.
Since there is no common sense today, your best tactic is to say nothing. Keep your mouth shut until the parents arrive and determine whether or not you need a lawyer.