I'd contact her the family, and say that you're really sorry, but that the college student did this without your knowledge, and the arrangement doesn't work for you - you need someone who is a legal adult and can do the job full time.
I wouldn't make it about the license, because then you'll have her coming back when she's sixteen, and a sixteen year old with a new license is a very different thing than a college student with driving experience and their own car.
I'm curious what the college student was paid, though. Did she really get $15 dollars an hour for the after school sitting? I would expect her to get more than the teen, in any event, as she's providing the use of a car.
The appropriate way for the college student to handle things would have been to ask you if they could subcontract the job out for a few hours a week, and then pay the teen out of their own salary, *if* you agreed.