[C]herry[B]lossom
You need to talk to someone. Think about who you trust the most (that's older). Normally the hardest part people find in talking to someone is knowing where to start, so I suggest you do the same as what you did here.
Talking to your school would be good because you can do it in confidence there and then they can get your mum up to talk to her about, that way she'll believe what is going on.
You don't need to stand for his abuse, you don't deserve it, that's why you must tell someone.
I think that you need to talk to someone that yes is older than you and that you can trust. I can understand how you are feeling, but no1 needs to put up with that.