Just wanted to add a few thoughts here... if there are people in your town who own danes (you mentioned you play with them at the dog park) maybe you can prove to them that you would be a good candidate to house sit when they need to go away, and of course house sitting includes all the fun and hassles of babysitting the dane(s). If the owners work all day, maybe they would hire you to walk and exercise their danes after school? OR, if you have a rescue or humane society nearby, you could start with volunteering there, and maybe your parents would agree to let you foster a dane if one is surrendered. Then you would gain firsthand experience in what it takes to be a dane mom, and what to expect from a rescue dog when the time is right for you to get your own.
I have to mention that my own daughter, who is a freshman in college this year, is finding it very hard to spend the amount and quality of time with HER rescued dog that she originally spent with him. She now has classes- She lives at home and goes to a community college (because of the dog); has a job, and of course wants to be out doing stuff with friends, going on trips, etc. As it is, we paid for his vet care, cherry eye removal, and foot the bill for all of his food and care, as she would definitely not be able to afford that on her own. In our case, it is a good thing we love Jesse, because when she transfers to another school after next year, there is no way she is going to be able to take him along if she lives in a dorm, and I seriously doubt she will make enough money on her own to be able to afford to live somewhere else and afford his care while going to school. The biggest issue with Jesse is that he was a seriously abused and neglected rescue, and bonded very hard with Kim, so if she is away overnight, he won't even eat, and paces constantly til she comes home.