You've gotten some great advice so far, and if you're going for a trifecta, point out to her that colleges and scholarships actually expect you to do more than just take hard classes. They're looking for time management skills like the ones you'd have to learn by balancing school and a part-time job. Most scholarships come out and ask why you didn't work if you don't list some job.

Agree that you'll quit if it does start compromising your school work, and that you'll take precautions before you even get the job so it doesn't become an issue. Unless you get a very understanding boss, it will be an issue--with either having to work a holiday or on a test night.

Perhaps before you commit yourself to a bunch of expenses, you'll work for a few months and save your money and then start asking about the phone. I can see she'd be concerned you'd extend yourself and then she'd be torn between wanting you to quit and having to take on the bills you've accumulated.

Your mom is looking out for your best interests, which is what she is supposed to do.
But I think it's great that you want to start taking care of your own expenses. That being said, holding down a job and staying on the honor track can be a lot to handle.

Maybe you could suggest to you mom that you take a part time job one day a week (like Healthy11 suggests) on a trial basis. If you're able to keep up the grades and the job and are not showing signs of stress, then you can keep the job.

I had my first job when I was fourteen but knew I would have to quit the moment my grades started to drop. Good luck!

I don't know what grade you're in, but in my state, teens have to be at least 16 years old to get a job, unless they're working in a family business. If you're 16, you're not a freshman, and so you should already have at least a year's worth of grades and "academic history" for your parents to evaluate and see how you're managing. If you've maintained good grades, and feel like you've got extra time, I would not see a problem in trying to find a part-time job, unless there could be transportation issues. (Do you drive and have access to a car, or are you close enough to walk to a job? Maybe your mom is worried about how you'll get home at night...)

Honestly, finding a job may be difficult in the current economy, but my teen got one, and quite a few of his classmates have, too. We told him that if his grades dropped, he would have to quit the job immediately. He was fortunate to be hired by a local movie theater, and so he generally works weekends, which doesn't really interfere with his nightly homework or tests. One of his friends teaches swim lessons and lifeguards at a local pool on Saturdays. Another works for a small ice cream shop, and again, weekends are their busiest time... If you can find a job like that, not only will it help you earn a little money now, but being able to balance school and work looks good on college applications and resumes! I hope everything works out for you....