The best thing that you can do is be real. And Honest with her, too many times, parents aren't wholey truthful with their kids and that drives walls between them. Explain to her that you know that this is something that she needs and its hard for you to let her, but you do because you know that she needs this experience. But on the same note, explain to her that you need to know that she's going to follow through on her word if you're going to let her have her freedoms. Try not to get bitchy sounding or bossy, just explain it to her like you really remember when you were her age. <br />
Above all... Be very up front about safe sex and birth control, She is human and that's going to happen anyway, so you might as well start drilling that into her head too, especially now
I'd be more worried that my 15 year old daughter has been putting her head up peoples butt's. But no I don't think you're being too harsh.