My SIL turned 15 in June & my sister is 18. I was there with both of them at 13.

We are real lucky with SIL in that she is pretty happy & confident that she doesn't need a boyfriend the way most girls think they do at that age. She's had a couple, but if she doesn't agree with something then thats it!
She's lucky I think that she's grown up in a small town being one of the very few girls & has gotten to know quite well what boys are like. She also has 3 brothers.

My sister on the other hand at that age was extremely defiant, rude, obnoxious (sp?) & was going to parties & got caught breaking into a holiday house with a bunch of mates for fun. She was 13 when they did that!
She also isn't too fussed on boys & has her first serious boyf now at 18.
Mum let her get away with alot though. They live in a reasonably small coastal town & mum bought her business when my sister was 11.
So from 11 on she didn't have very much supervision. I remember at 12 she told me she was going to a party down at the weir. I told her to wake up! No way would mum let her do that! Sure enough she did. I was appalled.
She came & stayed a night at my house when she was 14 & got a phone call on my house phone at 8.30pm. It was 2 boys wanting her to meet them around the corner about 500m away & go walking the streets with them! Over my dead body!!!
We lived in a bigger town about 20 minutes away from Mum. She wasn't impressed that I said no. I just told her that while she was under my roof she would abide by my rules so it wasn't my responsibility if something happened to her.
I told her they could come talk to her out the front til 9pm. I let her have her 30 minutes of privacy then went out & called her in. As I said, I was not going to let something happen to her in my care & she wouldn't have risked it coz I lived behind the high school. She needed somewhere to go when she wagged. & I let her. At least I knew where she was.

I have made sure I ask them questions & listened to their answers & just let them know that I will not tolerate crap, but I am here to talk about what ever they want to talk about.
They both tell me everything. I also give them advise. If my SIL came to me about sex I'd talk to her about it, then I'd let her know either she should go talk to her mum, or that I'd be letting her know.
MIL & FIL also have faith in me & trust that if there is something she can't talk to them about that she will come to me & I will respect her privacy depending on what it is.

So, sorry for going on & on, but IMO I think just make sure that you & her can talk openly & honestly, so that if she does get into trouble of some sort she can come to you. Try not to judge the things she does. & when she does silly things don't have a go at her, just maybe explain why you think she shouldn't do them or why she should be careful.

HTH.