I Am Experiencing Child Abuse?

Okay, this will be my first time telling anyone most of this question, so... here I go.
It started a long time ago, farthest back I can remember being abused was about 6, big blur any farther back. My parents would both spank and use a belt to whip me, a very, very hard leather belt, folded in half.
Once in 5th grade, my "father" was picking me and my brother up from school, to walk home, except he had this motorcycle that he would ride. As it was after school, we had our backpacks on, he took my brother's backpack, and so I lightly tossed mine over (I was in 5th grade-couldn't have hurt at ALL) and it hit him, in the back, and I remember snickering, because it was the first time I ever did something bad, even on accident (no, I'm no Jesus, I had lied, etc. before) and he out and slugged my face. It hurt, really bad, and I was trying to hold back the tears. Luckily, one of my friends had seen what happened and told our teacher - I got called out the next day and told her "It was just something I had to deal with." along with being asked by my friend if that was my dad. I've never told ANYone, friends asked, and I just told them that it was something about school. Over the years since then, I just tried to be the "perfect little child" that they wanted, but, I'm imperfect, and so it was beating after beating after beating. He has even threatened me that he was going to "send me away to a reform school if I couldn't behave." And I was considering trying to get him to send me away, while curled up in the bathroom, the only room I have access to, with a lock, but thought of who would be there: future thugs and criminals of tomorrow, and it would be more of the same. My other two siblings are NEVER treated this way - never threatened or even touched. I've considered running away, I've considered suicide, but I have never actually done anything. I'm now a 15 year old (to-be) Sophomore in High School, the quietest kid you know, and about 30 lbs overweight, I believe to be because of what I've been through. Please help, and thank you so much for even going this far, by reading this. And by the way - Alias ForProtection isn't my real name.