My mom won't let me get a guitar?

My friend has a guitar and sometimes she brings it to school. She let me play on her guitar and I really like it. I decided to get a guitar myself and learn how to play it. I ask my mom if I could get a guitar and she said no. She told me that since I was already going to school, I couldn't learn it. I told her that I already knew most of the cords and how to tune a guitar ( from my friend). I just wanted to play. I could go to my friend's house like almost every day to learn and stuff. I knew what guitar I wanted to get. How could I convince my mom to let me get a guitar???