If the US schooling system is like the CAN schooling system, then the one thing I've learned in my years of it, is that the parent reigns supreme.
I was disgusted at my peers because of how immature they could be, and disrespect the teacher like a doormat. All it took was 1 complaint from the parent to the principal, then a shitstorm would come down on the teacher, whether morally justified or not. Parents could easily press for their child to a better/advanced class, even if the child was unqualified from the start.
Normally I don't advise this, but if it is just, get your parents to go to the teacher, then the principal. If neither works, hit the superintendent. The thought of all that time investigating and paperwork usually is enough to coerce the teacher to letting you in, simple as that.
If this option is viable to you, I would have to agree with the above posters though, in first being civilized and having a reasonable argument with the teacher yourself, and school administration if that falters. Don't bust out your parents unless you have to. Nobody likes *that* kid.
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Once you hit college-level, that trump card is lost. The ball goes right back to the instructor's playing field, unfair grading or bias remains with the instructor. The only exception is if a grave, unobjectable miscarriage is apparent, in which case you can appeal to the department and faculty (but still at a great expense of time and effort on yourself).