I have 2 dancing daughters but neither had 15 hours per week at that age unless some were on weekends or were just special practices leading into big events. Honestly, for a 6th grader to be dancing that much (presumably more as she ages) I would start looking at other education options. We have friends doing homeschool co-ops and online education. dont think of it as mom sitting at the kitchen table teaching chemistry!!
For now, you just have to do your best. What we do on late studio nights is eat dinner right after school with a "second dinner" after dance. I bring something in the car for them. We keep a backpack of school supplies to do homework during the short windows that sometimes happen between classes ( i'm very grateful my kids are in school that uses books rather than online homework because it makes things much more portable). Remind her that she needs to use any free time at school to plan for those late days. Any work she can do at school helps. It stinks to have to slip away from friends at lunch to go sit somewhere and do homework, but I reminded my kids and they chose a commitment to dance in that require sacrifices. Her school might even have an option where she can bring a lunch and skip the cafeteria entirely to do homework and eat in a lounge or something. Also help her develop long range skills to use her weekends and slower nights to work ahead as much as possible. Even things like making vocabulary flash cards in advance that she can review in the car will make test prep lighter.
Also, keep on top of dance gear and set it all out for the whole week on Monday. The focus on those evenings should be honework and bed, not running around looking for other things.
I'll try to think of more ideas!