Am I being reasonable, or is my mother overbearing?

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My mother signed me up, and paid for, a racquetball class that meets every week for five weeks on Saturdays. She did not ask me if I wanted to be in this class because it was filling up fast and she didn't want to miss a spot. I have no desire to start having my weekends be completely scheduled since my week is already filled with school and student government and the like. Plus, I'm not really interested in racquetball all that much. I told her that I didn't want to do the class since it is in the middle of the only day that I don't have filled up. She is angry because she pent this money, but my argument is that it was a)her mistake to spend the money, not mine and b)a sunk cost and should not be included in my decision making about whether or not I take this class. Any advice? I'm also trying to get a job as a lifeguard, this class would make that hard. (I'm an 18 year old senior in high school, and virtually all of my peers agree with me, but my adult friends and teachers don'