My sister "dropped out" of school at 17, and it was the best thing she could have done. Since my mother wasn't thinking of homeschooling (she was just concerned that my sister recover from her awful school experiences), she pretty much let her do what she wanted to do. And so, after a period of "decompression", a young woman who had never read anything throughout high school embarked on a complete study of anything Jane Austin had ever written. She also went 360 degrees from punk rock music to suddenly studying and listening to opera! After a few years of getting up the courage to try formal education again, she graduated from college summa cum laude.
So my advice would be to try unschooling - let your daughter take the lead. You might set some guidelines about excessive TV, etc, so she isn't totally "wasting" her time, but at her age she is probably ready to really pursue her own interests. Why not ask her how much guidance and help she wants from you? You might start her out by getting her a copy of Grace Lewellyn's (I don't think that's spelled correctly) Teenage Liberation Handbook. Left to her own devices, you might be amazed at what she learns.