My mom is so hard on me?

My mom keeps pushing me about my school work and its tough because even when I get only 2 B's and the rest A's on my report card she scowls me and says thats not good enough and she grounds me when I get more than 1 B! is she being to hard on me or is she doing the right thing?? how can I make her understand that I dont like being pressured the way that I am??