What do you do when your moral compass starts acting up?

Last night I went to my friend's house to visit and catch up.  He's still in high school, but I'm well aware that he is pretty wild. And it doesn't bother me all that much because he's been like that since I met him when I was in 10th grade and he was in 7th. But when I got there, a girl that I have known since she was knee-high to a grasshopper is at his house also and talking about things that I never wanted to hear coming out of any high schooler's mouth.
See, the thing is, I was sort of an unconventional high school student.  I never had any desire to drink or make any other too destructive of decisions. I didn't want to piss off my parents because first of all, my mother is SCARY when she's mad and second because I never want to feel the burden of disappointment from them.  I enjoyed (and still do) the trust that I built up with them and did not want to ruin that.  So I was straight laced, responsible, and very irked by people that did not feel the same way.
Well, not a whole lot has changed since then.  While I have established more of a wild streak (haha, for me.  This includes having a glass of wine maybe once a month), and I don't judge as much for people's unhealthy choices...my moral compass still likes to go crazy from time to time.
This was one of those times.  I haven't felt that uncomfortable in a while.  I felt like everything was wrong.  I'm aware that high schoolers do things they're not supposed to do, I was no saint or anything, but I really have no desire to hear about it.  And it's not like I can (or would even want to) participate in said conversation.  It's WAY too risky being an aspiring teacher to talk about such things and even being present gave me flash forwards of never getting a job anywhere.
I wanted SO BAD to tell this girl that first of all: what you're doing is not worth it. You're throwing a lot away every time you smoke or get drunk or even talk about it.  You're making a terrible reputation for yourself and that's not a good way to leave the small little town you grew up in.  Second: who in God's name are you trying to impress? You think I don't hear this every Monday morning in my classes from hot mess college freshmen that no one wants to be friends with? You don't impress me one bit. I don't care how many times you've drank or when you puked and passed out.  All you're doing is making yourself look incredibly classless.
I wanted to say that, but I just sat there and felt very small. Most of the time my weekend consists of watching a movie with my roommates, studying, and sleeping in. And I really don't mind it that way.