Parents pressuring me on grades, feeling stressed/depressed?

I've always done pretty well in school so when I got an offer to take an advanced chemistry class I decided to take it. However, I haven't been doing great in there. Currently I have a 70%! I pay attention in class, I understand the hw, I get extra help from the teacher, but I still get bad quiz and test grades! I really do not know what to do anymore... I've decided that I could use some more help on my lab analysis/conclusion because I usually miss those parts on the quizzes/tests. The problem is my parents, more specifically my mom. She gets so mad at me whenever I get a bad grade and it puts so much pressure on me to get a good grade (she considers an 85% or higher good). I've told her that it's an advanced class so she can't expect me to get perfect grades and the school I go to is a tough school that really focuses on education. I've also considered dropping out but my mom insists that I don't and instead raise my grade. On my last quiz I got a 70% which is 5% higher than what I got on my last and I was pretty happy that at least I raised it a little which is better than it lowering, but my mom was still mad at me! I'm not sure what to do anymore and the pressure has started turning into a small amount of depression. I've literally began to cry over my grades :( What should I do?