I think that its a very sensitive situation because once children become of age they are masters of manipulation. I have saw first hand how toddlers can manipulate their parents to get what they want, so of course by the time they are teen-aged this becomes second nature.
I totally agree with establishing a good relationship where your children trust you and can feel that they can talk to you about anything, but the fact still remains that if it is a situation where your child knows that telling his or her parents is not going to benefit them, what do you think they would do? I was a good kid myself, and I had that type of bond with my mother, but if I really, I mean really wanted to do something that I knew she would not approve of I didnt talk to her about it! Children are smarter than that.
Of course, these days its not the same as when I was coming up. Instead of jumping off the roof of the house and experementing with lighters and matches (as I did when I ws younger lol!) now teens are far more frequently exposed to guns sex and drugs at a far younger age as well.
As parents we should expect that our children wont understand why they are under such a lagre microscope. but once they realize that all of those friends that we tried to keep them away from are in jail or worse, and to their suprise they have become law abiding and drug free citizens, they'll thank you as well as respect you.