Ellipses, if she's already taking HS credit (my DD is now in her third year doing this as an 8th grader, as well-- so I know exactly what you mean about early graduation and age concerns), then will they be willing to allow her to spend the
class time doing an independent study instead of undifferentiated material? Or does she truly have to complete the core of this course so as to gain credit for a particular HS graduation requirement? Those are things to find out if you don't already know the answers.
I agree, being teacher's "TA" isn't any kind of solution to undifferentiated materials any more than asking the mainstreamed special ed students to act as "class custodians" would be. It makes me mad that GT students are expected to act as nothing more than servants to their classmates, when it's perfectly obvious how demeaning and wasteful it would be to do the same thing other special needs children.

Once in a while is fine (my DD enjoys leading review sessions, etc), but as a steady diet, this isn't teaching the concepts theoretically offered in the subject, as I'm sure you're already well aware.
If you can find out what the teacher's subspecialty/favorite subject is, perhaps s/he might be willing to oversee an enrichment project in leui of continuing the current situation.
Would you daughter be willing/able to complete regular class assignments/assessments AND do an enrichment project on the side? If so, I'd pitch that as a solution.
This kind of thing really burns my britches, too; there's no way to approach it without it seeming as though you/your child objects to being with the special ed students, and that simply isn't the case. It's the fact that it's a SCIENCE class, not a student teaching one, not a peer-mentoring one, not an interpersonal communication one. KWIM?
Good luck keeping your temper.