M.P.
When I was a teenager I was a good kid. I hung out with "bad" kids and went to parties where there was drinking and drug use but I NEVER did it to. I was secure in myself and didn't feel the need. My frends all knew I didn't do those things and after they realized it they never even asked me to do it to. I ended up being the designated driver when they had to much and keeping a few girls safe from some no good guys. So it is possible to hang out with these kids and not be influenced by them. Keep in mind I had friends of all ranges from those who were viewed by their parents as "good kids" to the ones whom everyone assumed were drug users. I even had a girl offer me a sip of her vodka laced OJ at 7 am when we were walking from the bus station to school. Ironically, this was the type of girl no one would have guessed would be drinking.
That said you are on the right track. I agree that she should be on restriction for breaking curfew. I also agree that you are right in letting her hang out with her. It shows you trust her and also gives her the opportunity to use what youv'e taught her to make important decisions with the safety net and guidance of her parents near by. This is great practice for her before she is alone in college. It will help her understand the importance of choosing our friends. Still, be diligent and watch for signs of alcohol and drug use, talk, talk, talk, talk, about her choices and actions and let her know that you will be there no matter what. Now is a really good time to come to an agreement about her getting you to pick her up if she ever makes a bad choice and ends up drunk and in a situation where she is in trouble. I suggest letting her know that if she is ever in a situation when she needs you, you go and get her and then make a pact to talk about it in the morning. That way if she is drunk you can let her sleep it off and you have time to calm down and think about the best course of action.
Teens need to practice making their own decsions so they can make good ones when mom and dad aren't around, this is a good place to start.