I think what bothers me is that you keep saying over and over you want A chickie. With all your research, you should know you have to have at least 2 or 3 chicks. One lone chick gets really stressed out and lonely. You keep talking about wanting a cute chick, but they grow really fast and lose that super cuteness within a week or 2. I'm not convinced you're prepared for the long haul of owning a chicken for several years.
The biggest expense for chickens is usually the coop. To show your parents you're serious, get some free pallets and start building. I know I'd be impressed if one of my kids took on a project like that. It would show me they were serious about their desire and willing to work hard towards their goal. If your parents relent and allow chickens, you'll need a coop for them anyway. They outgrow their brooder in a month or 2, so a smart person will get the coop built first anyway.
Also, don't let your grades slip at all. Study hard and bring them up as high as you can to prove to your parents that you can handle school and extra activities. Academics are the #1 priority in my house and as long as my kids are getting good grades, they can participate in after-school activities and hobbies.
Good luck.