My bands got this gig coming up in like 11 days, so I've been trying to work hard to learn and patch up every single song we're playing for it. But, at practices, I'm always ignored or "wrong" at whatever we are playing, even when I know I'm right. I feel very under-appreciated right now and I'm cotemplating leaving my band even though its one of the better ones in school.

I mean, I know I'm just a second guitarist (I absolutly hate the term "Rhythm guitarist") but I think I'm just as important as the first one, right? And just because I play a riff different doesn't mean I'm wrong all the time. I'm just so goddamned pissed off and frusterated because no one seems to respect me as a guitarist or as a musician.

What should I do?
From the little info you have given, they sound like dickheads.

I say go ahead and ruin thier gig. Either quit early, or quit right after the show.