Hello zooploop: I believe this is your first post, here on PC. (I see you also posted this in the Teens Lounge forum, which is fine. But since I'm an old goat... I'm replying here!)

Anyway... welcome to PsychCentral... from the Skeezyks!

I hope you find the time you spend here to be of benefit.
This is such a distressing situation!

No one can survive the kind of constant abuse you describe with their mental health intact. It is incredibly sad that your mother is brushing off what is happening to you. Unfortunately, I don't know as I have much of anything I can suggest as far as how you can make this situation better. You really need some "in-real-life" outside support. Is there a member of your extended family (grandparents, aunts, uncles, etc.) you could seek help from? Is there a counselor at school you could talk with?
Continuing to post, here on PC, can help too. The more you post, & reply to other members' posts, the more connected to the community you will become. Plus there are social groups you can join, & chat rooms you'll be able to enter (once your first 5 posts have been reviewed & approved.) Lot's of great stuff!
Hopefully other members here on PC will come along with some more specific ideas as to how to deal with your stepdad. I wish you well...