Hi W.,
I follow a weekly parenting article in the Albuquerque paper, and a Mom had a similar question some months ago. The advice seemed wise to me (My little ones are a long way from teen years).
He recommended parents avoid "micro-managing" their teens. Allowing them to set their own schedules, being responsible for themselves (chores, homework, sports, etc) is very healthy for teenagers, and helps them take on the responsibility that will be expected of them as adults.
In other words, as long as she is getting to school on time, doing all her work, getting chores done, meeting all her family's expectations, getting good grades, etc. Leave well enough alone. She is making more adult decisions, including when to go to bed, and how tired she is willing to be the next day. Grumpy? She's going to have to deal with the consequences if she takes it out on anyone else.
Hey, you're doing a great job raising a big family. Maybe as the youngest, she's picked up on the older ones' maturity, and is just making the decision to be more like them?
T