School stress, How Do I Get Rid of It?

I have been having a lot of pressure at school. Especially in Science because I'm getting a D- or F- and my mom expects me to get an A or B+. I do all the things they tell you to do. I scream into pillows. Punch/beat up pillows. I try to get rid of all the stress outside of school, but I still have this pang of stress. I just can't get rid of it. It has gotten even worse lately because I'm trying to be nicer to my siblings so I can't just punch my little brother all the time now. Also I used to excercise/stretch/dance in my room when I listened to music almost every day, but now my sister (who I share rooms with) is always in there so I cant excercise/stretch/dances with her there because then she would call me an idiot. I really think I just need as relaxing day away from school, but my parents will never let me stay home from school unless I am sick.