It can be done, it's not too hard, but you have to stay on top of your work. As soon as you get an assignment start working on it, and try to finish it ASAP. It really helps if you''re interested in what you're studying. This means you don't party too much or have much time when you're not busy.
Skiing> partying. Try working extra hard in the fall semester too, so if your GPA slips a couple decimals in the winter it won't hurt as bad.
I take 5 courses a semester (3 or 4 are studios, which are each 6 hours a week in class), maintaining a 3.8 GPA. I don't party much (this also saves a lot of money) and I work as a ski instructor on Sunday and Saturday at the really busy times. I'm on the hill at least 3 full days a week, and I ski every day, even if it's just a little line in the trees that I hike to near campus. Don't expect to progress as much as usual when you're at college, because you are focussing a lot of your time and energy into school, but I would say skiing can help you do better in classes, because it keeps you active, is an outlet for energy and a stress reliever, and you need all those things for your brain to function right.
Don't mind me.