And I was just going to post a response. . .
I was going to say that you could tell her she can quit her job if she thinks it best, but she still owes you money and she still needs to buy prom crap etc. The decision is hers, really (but you need to stick to not giving her money for all the stuff she agreed to pay for out of her salary).
I learned the value of money in h.s. not because my parenst made me work, but because if I hadn't worked, I wouldn't have been able to buy senior pictures, a senior ring, and all the nonsense that I thought I neeed at 18 but that my parents wouldn't (and honestly couldn't) pay for.
Some of my friends' parenst tried to teach them the value of money/work by encouraging them to get part-time jobs, but it didn't work becuase the occasion (need money to fill "need") wasn't there since the parents were still forking over hefty allowances and buying them any clouthes etc.
Good luck. Working part-time in h.s I had this strategy for balancing fun with my scheduled hours at the mall. I'd always never miss something big socialially over work (so if something REALLY fun came up, I'd call in or switch shifts) but I'd never not go in just because I didn't feel like it. Of course, this worked well because I had only a small group of friends. If your daughter is more popular, she may need a better stategy