How can I convince my mom to drive me to the barn again?

[SassyAussie]

Since February my mom has been driving me to the horse rescue that is about 15 minutes from my house, ever since the horse I'd worked with had been adopted in September, I haven't had a horse to work with, so she hasn't been driving me there. Last week, my volunteer director emailed me asking if I wanted to work with another horse, my mom said she'd think about it. She has really enjoyed not having to drive me there 2-3x a week. I have always thanked her for driving me there, and once 'my' horse was adopted she suggested that I drop myself from the program gracefully. The thing is, I LOVE going there, it made me really happy to be able to go and help out with the horses. She claims the reason she won't drive me is because of my sibling's soccer schedule, my D in Physics, and my behavior. My siblings don't have to be picked up from soccer until 5:00pm every day, we get out of school at 2:00pm, that is 3 hours of time she doesn't really do anything with except go on a quick bike ride. I have been working really hard to get my Physics grade up, both she and my teacher know that, I have been working with my teacher during my study hall every day at school. I have been on my best behavior too, I have done chores without being asked, not complained about anything, I have done everything shes asked me with a smile, I've been thanking her for every little thing she does for me. I have 'A's in all my other classes, and I've been working very hard and doing a good job. Really, I've met all of her requirements and been working to go past them too. She knows how much I love going there, I think she just doesn't want to drive me for her own personal reasons (it takes her time away from talking to her mom and brother). I have been doing nice things and getting good grades before I found out that I had a potential horse to work with.
How can I convince her to bring me to the barn again? I haven't asked her since she said she'd think about it. But I do things and rarely ask for recognition or thanks, without being asked. Usually I don't even tell her I've done something nice for her unless she asks me to do it, and then I tell her that I already did it.

Would it be unreasonable to ask her if I can take a working student position that is a mile from my house? We drive right past it on the way home every day, and I can walk home when the weather is nice. It literally takes 2 minutes to get there, probably less. There is a heated indoor where I could ride during the winter. I can't ride my friend's horses because they don't have a indoor, or a area where it isn't too icy to ride.