What do you do when you have a really hard teacher and you're trying your hardest but she still gives you a low grade?

Look at what kind of work you're doing. Anything that can be answered without argument, such as math, ( 1 + 1 is ALWAYS 2, for example) can be remedied with more studying time and and possibly a tutor. If it's a class where you write papers such as english, then try and compare your work with someone's elses to see how they faired. If you consistently are doing poorly in a class and you have ASKED the teacher on numerous occasions to help you, and you possibly feel that the teacher isn't doing their best to help you; then you have a right to talk to your parents and principal about the teacher in question. Make sure you do what you can to help yourself first, seek a tutor, find a study group, ask the teacher for help. When all else fails, consult your parents.