T.A.
A.,
It sounds like your daughter has a lot on her plate at such a young age. While it makes us proud to be able to list off the accomplishments of our children, we need to be sure that they are not going to buckle under the pressure. It sounds like something's gotta give and it is best if it could be something positive so that it doesn't end up being a negative. She may be under a burden but is not wanting to let you down.
Traditionally, we as parents, think of it being our job to TALK to our kids. My guess is that you have talked enough. Why not LISTEN instead? She seems quite responsible - something you have obviously instilled in her. Instead of telling her she is taking school less seriously, why not ask her how she is handling everything on her plate. It may take several conversations and you may have to hold your tongue ALOT!!! If you can't say something in less than 30 seconds to your teenager, you will lose their interest/attention and it will become a "lecture" or "getting yelled at" in their mind even though it isn't your intention at all.
Another thing I know about teens is that their "currency" is their friends. You start saying, "I don't want you to turn out like so-and-so" and you will lose her fast. Leave the other kids out of it. You are only responsible for her.
Tell her you aren't going to make a big deal out of it anymore. You are concerned but believe in her. She knows your values and dreams for her. Tell her that whenever she needs a safe place to fall, she has you and then don't bring it up another time except to ask, "would you like some feedback?" She knows right from wrong but, unfortunately, at this point, she has to walk it out.
Good luck - this is why parents get paid the big money, right? :)