#1. Quit smoking. Its definitely not gonna help make things better in the long-run, it seems like its wrecking your relationship with your parents and its gonna definitely screw over the rest of your ski season. I know this may sound stupid to you at this point, but there are better ways to make yourself feel better. Sitting at home and smoking isn't gonna get you outta this funk.
#2. Work hard to improve your grades. Take the time to do your homework, ask the teacher or that smart kid in the front row for help, and ask around and see if there is anything you can do to bump up your mark. Homework sucks, but its the only way you're gonna get through school, so stop handing in assignments late, study for tests, and actually spend time on the work you're doing. As soon as you start putting in the effort you will start to see results, I promise. And if studying isn't getting you anywhere and you are still struggling then ask your parents if you can get a tutor. If not, go to extra help sessions after school or during lunches. Showing the initiative will really help make a difference.
#3. Take the time to appreciate what you have. From what I can see you have a pretty special girl in your life, and although your parents sound like they can be jerks I think that at the end of the day they have your best interests at heart. It seems like the whole meeting with the principal was them trying to get you to step up your game academically - not them trying to make you even more depressed. Just try to think about the positives. I know its hard but maybe just take the time to write down what you DO have going for you right now (your amazing girlfriend, your skiing talent etc.) and focus on being grateful for those things.
If you can do all of that stuff I promise you life will get better. Just thinking on the positive side for a few minutes each day will do wonders to improve your self-esteem. If you can quit smoking, you'll piss clean, get your skis back, and improve your relationship with your parents. If you can manage to boot your grades and show some responsibility and initiative then your relationship with your parents will get even better, and you won't get so down on yourself. Getting good grades feels amazing, and studying/doing homework is definitely a great distraction from moping around. Also, maybe you could try to strike a deal with your dad, something like "if I get a b+ average in school then can I start to do some small ski competitions?" Who knows, maybe he will be more inclined to let you compete if you straighten yourself out a bit.
Sorry for the novel, but I hope it helps.