I have a group of really close friends. They aren't necessarily as close to each other as I am to them, but they're the nicest people I've ever met and I love them to death each in their own way.

My problem is that my friends, like all human beings, always do things which I don't approve of. It's not personal choices like religion or appearance or whatever. For example, one of my friends is a really smart girl, but she is kind of lazy and I'm watching her get low grades and get into trouble for it, when if she stops slacking she could do amazingly well. She's always creating drama with other people that is so unnecessary.
Another friend is such an intense person, I can't tell her anything about herself without her getting ludicrously angry at me. I tried to tell her the other week to take a break from studying for an hour because she was running herself ragged with her head in her books and seriously stressing herself out, and she practically bit my head off!
My other friend is just always making bad decisions in life, and I want to help her by telling her what to do, but she's also the most stubborn person I've ever met, and that's not a good combination at all.

So, I want to know where to draw the line with my friends. Where are the boundaries? I feel like I should be talking to them more about their problems because I care about them and I don't want to see them get in trouble when I can practically see trouble coming. But at the same time, I'm not their mother to be telling them what to do and preaching to them about anything at all when I'm far from perfect in my own decisions!!

What do you do in these situations?

What you do is more then enough! :)
It differs from friend to friend!
Some of my friends would love to see people helping them even if they turn people down.
While others will want to be LEFT ALONE and not talk about it at all.

I would talk to them once, twice or even 3 times to tell them how i feel about it!
After that I will leave it if I saw no improvement for like 1 week or 2 to see if she/he really thinks about what I said or not.
If they start thinking about it and want to try you have to be there for them to be their "support" and if they still don't care tell them you care what happens to them and you want them to be happy so you can be happy!

I am not sure if this helps but try it if you want to..

Being a friend includes an element of being a "mom" too and that includes advising them and guiding them, but you should never force them to accept your views. But as a friend, you should at least "try" advising them. If they don't want you to, then you can stop. At least you tried, right?

As long as you tried and they took it personally then their is nothing you can do about it.

If they're very close to you and you care about them very much, you should never give up. Try and try and try forever until you see there's NO hope.. After all, they do mean alot to you and you don't want to be the one telling them: "I told you"! Talk to them, advice them (but do it at the right time- when they're not doing what you want them to stop doing), and show them that you are very consistent and serious about it.. If that doesn't work, you will atleast be satisfied that you've done something about it.