I brought my son with me to China at the same age, but unlike your daughter, he really didn't want to be here. However he did agree tp stay for two years and could have gone home at any point after the initial first year, but he did chose to stay per the original two year agreement. With that being said, here's how I managed it:
1) He flew over with me on a tourist visa.
2) I enrolled him in a Chinese language program at one of the local universities. That enabled him to get a student visa which led to a residence permit.
3) He lived off campus with me for the first year.
4) When we moved to another city, I enrolled him in another Chinese language program and he stayed on campus his second year.
...would she have the option of attending a high school and/or language school?
If your daughter is still a senior, then you could enroll her in an International school, which may be costly, unless you are an employee there. In which case, in addition to negotiating your salary and benefits, you'll have to negotiate her tuition, bus fee, lunch costs, book fees, lab fees, extra uniforms and such (so you don't get socked with a huge financial burden for a bunch of little extra charges that add up really quick.) I would NOT recommend that you enroll her in a Chinese high school. I can't think of one case where that
turned out to be a good idea.
Another way around it, is to homeschool her. Sign her up with an umbrella homeschool organization. They will counsel you on which credits she needs in order to graduate and in the meantime, you can enroll her in the local university's Chinese language program and add any courses she takes to her transcripts, as well as, qualify her for a student visa.
...will she still be considered a dependent at age 18...
Nope. Sorry, I tried to go that route too, but it was a no go.