I wanted a part-time job when I was that age and my parents were not crazy about the idea but eventually relented - on the condition that I kept my grades up. If my schoolwork and grades slipped, no more job. It was only 10 to 15 hours a week so it wasn't THAT much of a time commitment, but it did teach me how to prioritize my time. On the school nights that I had to work (it was no later than 9 pm) I did my homework right after I came home from school because I wouldn't have time to do it later. And it gave me a taste of working in the real world with the public and up until then I felt I had been relatively sheltered. My stepsons are 17 and 18 and both are working at their local Burger King (the older one has graduated high school and the younger one is 11th grade) - they both pull 32 hours a week, and at least with the one of the them still in school, it doesn't leave a lot of time for getting into trouble (which they had before - idle hands are the devil's workshop!).
I know it's difficult, but don't keep her from growing up if this is something she wants to try. If she's got that much ambition, don't hold her back. There's way too many posts on here from parents who can't get their teenage (and older) kids to do anything - done with high school, won't do college, won't find a job, won't even get a driver's license, etc. Seems they are still "enjoying their youth" but for a little too long if you ask me. At least she has some drive. At that age, I couldn't wait to get a part-time job, get my driver's license, and then finish high school and head off to college! And I still had plenty of "youth" left to enjoy! As long as she can handle a job along with school and such, let her go for it. Like you said, she can always quit if it doesn't work out. However, I would not consider what she earns as "household income" - my job earned me my money to do with as I chose, whether it was to put in my own savings, or spend. I was not expected to turn it over to my parents to help them pay the bills (I was responsible for keeping the gas tank of the car filled though).
In my experience, kids that are not allowed to try things, and not allowed to attempt some grown-up responsibilities, or are not expected to, end up either losing confidence in themselves, or becoming resentful and eventually rebelling. I don't like seeing kids grow up too much too fast, but I don't like seeing kids kept from growing up when they should (I see this in my husband's ex with their kids so it's a bit of a sore spot with me). Don't let the message be from you that you don't think she is capable of handling it.