For me high school wasn't a struggle, I won't say I'm brilliant but school was easy for me, I was bored most of the time and graduated early. I got out of class early to ski, went skiing before/after I had class (we had free periods at school, some days in the morning, some in the afternoon). I took advantage of the free periods every once in a while for stuff I knew I didn't have the motivation to get done on my own time. If it was something I could do quickly and easily, I'd do it in the evening when I got home from the mountain. I'd study for tests with the notes I took and added notes that I'd write in a way that I understood later, sounds nerdy but that's what worked for me to ski and keep good grades.
I can only give advice on what I know which is mostly time management and just buckling down when it's really needed. Clearly you can't just stop studying, and if you stop skiing, you'll be miserable. What I'd do is take advantage of programs your school offers, tutoring, free periods, study hall, anything to get the grades up to a good base line. Then I'd work on figuring out what you can change to keep them up. The more you work your ass off to get the grades up now, the easier it'll be to keep them up when midterms and finals come around. So try your best to prevent any avoidable slip ups, that way if something comes up like a late paper, you'll have a higher grade to begin with and it won't take as hard of a hit. As far as tests, those weigh higher so that would be one thing I would recommend keeping at a high level. Even if that means going out to shred for a one or two hours, it's better to study for the test than be in hot water and have your parents pissed and your grades in the shitter. 100% worth it to study in a way that works for you, notes, reading, study groups with friends, just make it work, get 'er done.
You can't be perfect, but if you want to ski, it sounds like you have to make an effort for school. It is what it is, might as well work for higher grades so you have more options in your future.