You're fifteen years old, and whether or not your realize it right now, you're still a child. They're only doing what they think is right for you. In a few years you'll come to realize that. They'll respect you a lot more and give you more freedom if you treat their decisions with respect... without complaining. It'll show you ARE more mature.
If you want to be treated like an adult, then you have to act like one. You have to learn to study on your own, and listen to authority figures. When you get older you might need them, and then you'll see that they did the things they did for a reason. Sometimes no is no.
You're just 15. It's normal for you to feel that way toward your parents. I believe you'll thank them for what they do to you now when you grow up. You know, for me, the more my parents control me, meaning the more they love me because the more they care about me. Trust me, they'll let you on your own when they think you're ready.
You're Fifteen; you aren't an adult. You're not going to be in control of your life for a few more years, and when you are? You'll wish you weren't. You're going to have to learn to study on your own, and to listen to the athourity of someone, even if you don't like it. If you can't respect them over a simple decision/argument like that, then it's really no suprise they don't treat you like a capable adult. You called them bad parents for changing their mind? Bahh; well, as Audrey said; they've got their reasons. Just grin and bare it so that you can earn their trust and respect and such.
...Wait... 'You're smart' = bad parenting? I'm too tired for this.
You're 15. You've parents are supposed to control your life. If they didn't , then they'd be bad parents. They've probably got their reasons for not wanting you to go.