(for legal advice, skip to bottom)
Don't try to scare her into staying, I don't think that will work.
Why is she running away? if you can identify that, you hopefully can teach her not to run away from her problems.
If it is a domestic issue (something within your family) don't let her have her way, next time she has a problem she will just run away again.
does she run away for a night, then come home? or will she never return?
is it just to gain general attention? is it running away from a problem? is it running away to cool off, or think?
Why doesn't she like to go to school? is it boring? useless? is there a very specific reason, or it just "I don't like it"?
You might be able to say that next time she runs, you aren't coming for her, and that from then on she is on her own. The problem with that is if she ignores it, and runs, then comes home. You have to let her back in, but at the same time that shows her that she can leave whenever and come back whenever.
At 15 yrs old, she probably isn't ready to live on her own yet, I know I certainly wasn't.
Honestly I don't have any good solid advice for you. You don't have to answer the questions I asked, they were more for your thoughts. One thing I do know is don't try to force her home using the law, none of their possible solutions are good.
Go to a professional who deals with teenage psychiatry or counseling.
As for legal advice, she is held accountable for her actions, regardless of age, the question is, are you held accountable for her actions. And the answer is until she is 18, yes. If she runs away, and you do not attempt to retrieve her, child protective services will probably take a look at your family, and see if you are unfit to raise your children.
