You know, it's hard to say. I could tell you you're focusing on skating too much. I could tell you school is more important. Yada yada. But, I don't know your whole situation.
I was a bad student. Not un-smart, mind you (I once got yelled at by a college rep at a college fair when he found out the discrepancy between my ACT score and GPA), just didn't apply myself. I had a lot of outside activities, but they were NOT the reason I didn't feel like doing my homework; if I did nothing but sit at home all night every night, it wouldn't have made me do my homework. Perhaps that is why I can't recall my mom ever "punishing" me by taking away my other activities, because she knew I was lazy, not overworked.
I get that you need to do your homework, and I'll tell you that for sure. But, I do think your mom is being unfair to complain about your skating. If she wants you to not skate, then she needs to say "no, you're not skating, we're not paying for it, we're not taking you there," and make that the end of it. But if she wants to allow you to, then she can't complain about it every minute. My mom was like that sometimes and it drove me nuts. She'd say, "yes, you can spend the night at your friend's house" but then the whole drive there she'd complain about it. THEN JUST SAY NO! It's no good to tell someone they can do something and then make it very clear that you didn't want them to do something but didn't have the chutzpah to say no. Granted, some kids are whiners and the parents think it's easier to give in, but it's still the parents' choice to do so, and trying to guilt-trip the kid doesn't do a lot of good, at least as far as I've seen.
So yes, I've said it. When it comes to complaining, I think your mom was in the wrong. Also as far as when it comes to holding you and your brother to different standards. I understand why people do it--once someone sets a "pattern," they're held to it (which is why people are often advised not to set a pattern of being an overachiever--whether at work or school or elsewhere--lest they always be expected to be perfect). I understand that in some instances, people just work to the best of their strengths and their strengths aren't equal. And yes, perhaps you are just smarter than your brother. But when it comes to effort, I'm sorry, but there is no discrepancy in strengths. I tell you this AS someone who's prone to be lazy and unambitious. There was nothing stopping me back in high school from getting stuff done as well as the next person. I had no deficiency of intelligence or physical ability or maturity or what-have-you...just lazy. So I give him no pass there on not getting stuff done, either, and she should totally be holding you both to the same standard there.
Mind you, I'm not trying to take sides. I don't know you, your mom, or your actual situation. But I saw a lot of people getting down on you, and I wanted to point out that I don't think you're 100% in the wrong here.