You need to go to a teacher, school Principal or Vice principal or counselor, or a minister at your church and tell them this. All of these people are required by law to report it to the police if you tell them this. The first time, the police or child welfare worker might only come talk to your Dad and when they leave, it might be worse for you at home. But if you continue to report, they will be forced to force your dad to live apart from you (either you will be moved elsewhere or he will temporarily) and he will have to take parenting classes or spend time in jail or both before he can get back together with you.
All of this will be very upsetting for your mom. She might be so upset that to prevent the separation, she lies to police and defends your dad against your word. But you need to keep telling the truth until telling the truth sets you free, because the other part of the truth is that it is really not okay for someone to keep beating you. Your safety matters. Your mental health matters. You matter.