Saw a movie once that demos this, apparently lots of kids have a rope hanging in their closet. Kid gets up first thing in the morning and wraps the rope around his neck and dangles until he passes out and falls crashing to the closet floor. Mom comes in, sees him just waking up, and does what many parents do, turns away because she doesn't know how to cope.
Personally, IF given the choice, I'd rather my kid smoke pot till they pass out than by asphyxiation. You pass out from lack of oxygen and basically kill off your brain cells in the process; the same thing happens as what happens in carbon monoxide poisoning, which leaves survivors as brain-dead as broccoli stalks. It's definately one of the most stupid possible ways to get "high."