Just a question. Is this girl on birth control? If she's not, it is time to take her to the doctor. All you need is for her to end up pregnant, which will happen if you aren't proactive about this. I'd also find out who this boy's parents are and talk to them. Hopefully they would back you up on this. (Unfortunately not all parents are good parents though.)

Clearly she is testing the limits. Sit her down. Go over the rules and see if you can get her to understand that if she simply stays within those boundaries, life will go so much easier for her. Set the structures up ahead of time rather than just being reactive to whatever she is doing wrong. Catch her doing right -- a lot. Keep her very busy with structured activities. You know: idle hands and idle minds . . .

And I'd be really wary of leaving her alone at home. Some kids just can't handle not being supervised without finding mischief to get into. We'd love to be able to give our teenagers more freedom, but some of them are just not emotionally equipped for it, and they can get into some seriously dangerous activities.