A good many cities have anonymous hotlines now days, and if you have a name and a script of what seems like is going on. ( it depends on the size of the town and the mood of the person answering the phone )
As for a good throw down!
Misdemeanor, maybe a class c felony, $188°° fine and maybe there Dr bills if any.
Done it more than once, I was just smart enough to make them throw the first swing and let it connect as proof. You just need to know the right insult to make them swing. 
Most of the time laughing at them does the trick.
There is no US rule saying you can't take photographs of a minor, you don't even need permission to do so, if done from or in public areas or access frommm yada, yada, you can reed the rules in the " Photographers Rights "
Perception is another story and the one that causes problems.
A ticked off boyfriend, jealous girlfriend, some adult hear her bragging to one of her friends.
- An anonymous phone call, to a hotline or the PD front desk.
OP, you stated your concern when you started this thread, if it was not a concern, then it would be a (what).
You even mentioned the fact there would be no bathing suit or nude shots involved.
If they went through your system and found this, and you FB imagine what they would think.
Worse yet if she is the only one in that age group you FB with and far be it, should it not be about the business of photography. If any of it in the slightest looked like flirting.
Common since should have told you to involve the parents after the first hello or two.
That you have not yet done so and still state that your are going ahead with your plans could be more for them to read into. ( as you have no business game plan in play to help show the fact you are on the level ) perception for you so far is not in your favor.
Look at IT from the other side and see there thought path.
Someone out there new to the law side, looking to make a name for their-self and you the perfect pawn.
You want to make a portfolio for senior shoots and want to show your best work.
Photographer, why don't you photograph one of your family members or a friends teen?
- What so special about this girl that you have chatted her up on facebook and flirt with her where she works.
Do you have a thing for her or what- Photographer??
And man careful how you answer there questions as they are designed to have no correct answer.
Is the above an extreme? Yes it is Very. -Would it happen? That only time will answer.