Ah, something I know a lot about... troubled teen. I was one and then I worked with them for five years at a job and was a foster parent. In a way, I still help by unofficially mentoring.
Anyway, Help For Troubled Teens is a site with options for parents to help turn a kid back around once they've stepped onto the path to hell. If you thinl your teen may be in trouble, check out this helpful list of common warning signs. I had the truancy thing down pat, completely changed my set of friends and became very aggressive. Without intervention, who knows what path my life would have taken.
Helpmyteen.com recommends long term treatment facilities rather than boot camp type IN YOUR FACE kind of solutions. They don't deal with hard core drug or legal issues but rather kids who are on the verge of making those disastrous choices.