I think you need to request that the school differentiate in math at the same time the rest of her class is studying math. I know it's not as simple as it sounds from what I've written, but I think it's worth fighting for.
We've gone the differentiation route with this school, on more than one occasion, and it has never worked out well. It was always something promised to us as an alternative to a grade skip, and it was never delivered in any consistent or meaningful way. Also, it would be hard to argue for differentiation when that same need can be provided through G/T.
The thing that would really concern me about the current situation is what might happen during science and social studies that's spontaneous and not part of the lesson plan or perhaps not included in the textbook. This happens all the time at my kids' schools in science and social studies etc - during classroom lessons kids will be exposed to something new, questions will come up, discussions will go off on slight tangents or include new info or direction etc. Those things might never show up on a test (or they might?) but they are a big part of being part of the class and also potentially play a part in inspiring a love of social studies or science or just learning in general that you might not get from reading a textbook.
Actually, this is why we'd rather have her textbook at home, where DW or I could set aside some time each week to discuss the current material. She gets all her questions answered, and I know that whenever I'm in the mix, I take her well beyond the written material. DW has already assigned social studies to me (I'm a history junkie, and she's not a naturalized citizen), and some of the science will get kicked my way, depending on the topic.
I'm also curious what specifically is holding her science/ss grades down - is it accuracy on worksheets she does in class, is it test grades, discussion grades...? I think that if you looked at the specifics of how it's impacting her grades that might help in advocating.
Test grades. I'm sure it's because she's never there for discussions, and doesn't get enough time to read on her own. Her exposure to the subject is pretty much limited to flipping through the book while doing the worksheets. That method only fosters the collection of a few memorized facts... not learning.
