Why is my stepmom being so mean to me?

I am 16 and I have been having some trouble. Last night when I asked my stepmom to help me with an Algebra problem, she stood up, threw my paper down, and then shoved me into the wall and told me she doesn't have time for my petty problems. I have a bad habit of not making my bed in the morning, so today when I got home from school she grabbed me by the arm and dragged me into my bedroom and then she kept hitting me over and over. She has been doing these types of physical things lately. Why is she doing that? I try really hard to be nice to her. I can't tell my dad because he will say something to her and then she will probably kill me. My real mom lives in another state and I never talk to her. What should I do? I have bruises on my arms from today and I'm scared my teachers are going to ask how I got them. I can't wear a long sleeved shirt because it is very warm where I live. I know I'm 16 but she is a lot taller and bigger than me so I can't really fight back. What should I do???