I smoked for a few years and quit cold turkey. Mainly I did it because I was getting sick all the time - respiratory stuff.
If I may...
Cigarettes contain many harmful chemicals. Tobacco, in and of itself, isn't very addictive or as harmful as the additives in cigarettes. Look at pipe smokers and cigar smokers. These types of tobaccos are relatively pure, meaning nothing is added (hence the need for a humidor). Pipe and cigar smokers don't tend to have the same problems of cigarette smokers. I know it's not very scientific, because most pipe and cigar smokers don't do it constantly every day, so the amount of smoke inhaled isn't comparable. My point is that the cigarette companies put stuff in the cigarettes that help preserve the tobacco. No smoker will willingly smoke stale tobacco - they only do as a last resort.
Winston brand cigarettes lost a lawsuit many years ago and one stipulation of that was they could only use tobacco. No additives, no preservatives, just tobacco. As a result, getting a 'fresh' pack of winstons is harder to do. But, they aren't nearly as addictive. I was smoking Winstons when I quit, cold turkey. The only time I craved a cigarette was at the times I normally smoked - but it wasn't a physical addiction, it was psychological. I'd want a cigarette when I'd climb in my truck, or after a meal. Once that psycho addiction went away, I didn't have a need for cigs anymore.
As for smokers themselves, you face a big hurdle. I'd say it's worse than confronting a drug addict. Cigarette smokers are constantly in denial. They don't think they stink, they don't think it bothers or hurts anyone around them. They don't think they get sick because of the cigarettes. They just don't think cigarettes do anything bad. Getting smokers to realize that their addiction is harmful, not only to themselves, but to their friends, family, and nearby people also, is the toughest thing to do. Just as an example, my brother-in-law and his mom both smoke. My brother-in-law and his wife just had their second child, a boy. When either of them would pick him up after they had just smoked, the boy would just fuss and fight. Neither of them could figure it out. When I pointed out that he's suffocating in the hazy stink of cigarettes, they'd almost get angry with me. All I could do is point out what was happening, because he didn't fuss and fight when any of the non-smokers picked him up (unless he was hungry, that is). My brother-in-law and his mom are two perfect examples of the smoker. While my brother-in-law does admit that it causes problems, he choses not to do anything to stop.
Lastly, I'm not sure what state you live in, but several states have laws against smoking around children. Here in Texas (as well as about 5 other states) it's actually ILLEGAL to smoke in the car with car-seat aged children in it. I realize that you're not riding around in a car-seat with your parents, but there's a reason the law is there - SMOKE IS HARMFUL and kids who are exposed to it early have a bigger risk of lifelong asthma problems, as well as stunted growth and reduced brain function.
So in conclusion, you probably won't be able to do anything to get your parents to stop smoking. If you do get into some trouble at school, like bad grades or bad conduct, you could blame it on the smoke - say that you don't sleep well, and your grades suffer, or kids at school are always harassing you and you act out... You can't physically slap your parents in their faces about smoking, but sometimes if they see ill-effects on their kids, they'll think twice.
I've gone on and on, but I really hope that you can get your folks to see how harmful it is to you that they smoke. In the very least, you may convince them to just smoke outside, or in the garage or something. It could be that they don't realize they're picking cigarettes over your health.