I study but i still get bad grades?

Whenever I get back from school, I take out my binder and do my homework. I don't play video games because even though I have an xbox 360 I don't have any games because I sold them all. I study at home and at school, but I still get bad grades. I get good grades on cultures, but everything else isn't... show more Whenever I get back from school, I take out my binder and do my homework. I don't play video games because even though I have an xbox 360 I don't have any games because I sold them all. I study at home and at school, but I still get bad grades. I get good grades on cultures, but everything else isn't good. I don't know if it's because I'm just unlucky or if I'm not studying the right way. It's really stupid because I have a friend who plays 8 hours of video games every day and gets straight A's and I don't play video games at all and I study at both school and home, I do work in the summer and I still get a lot of C's. What is wrong with me?!!
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