My mom is angry all the time?

Ever since I was twelve, I could tell my mom had an anger problem. For years I just watched and watched as my mom got angrier and angrier. Now, my grades are dropping, and my mom freaks out at me at almost any moment. Every day I get called a lier and other names. It really depresses me that my mom is calling me the's and I really dont know what to do. My dad is really calm when my mom gets angry, and so have I but I can't really take it anymore. Everyday I just get dissed by my own mom, who I cant trust telling anything without the fear of her flipping out on me. In public my mom is really nice and sweet, but once were home, she starts yelling at me about my grades, calling me stupid and retard. If she's not shouting at me, she's shouting at my dad or my brother I DON'T KNOW WHAT TO DO. :(