YABU. I know teenagers from the most supportive families can go off the rails. DN is 14 and at a leading independent school, at BIL's insistence, not ours. I spent an afternoon looking at some of her friends on Facebook - all at the same or similar schools (Chelt Ladies, Eton, Wellington, St Mary's Ascot etc) from very supportive and attentive families, though obviously not web savvy ones - and felt like taking a shower.
Most teenagers do dreadful things, there are undoubtedly strong cultural pressures pushing them towards underage sex, smoking, drinking and drug use. Some children are beyond tutoring and will end up failing their A-Levels regardless of the school they go to.
BUT BUT BUT BUT BUT just because most families can't change some things about teenage life in modern Britain it certainly doesn't mean that their love and attention is wasted. If you give a child structure and support, help them develop goals and plan for the future then you've done them a huge service. I'd be prepared to guarantee that children with early bedtimes, good diets and lots of bedtime stories will probably, on average, turn out better than children who don't get these things.
The tone of your post seems to be "well, whatever you do, you don't know how they'll turn out". So why do you even bother then? Why not let them choose their own bedtimes and recreational activities? Let them choose what to eat, what to wear, which school they want to go to and when they want to go. They might end up dreadful children but at least you won't have a "big shock" when they turn out that way!