Same situation here, with DD10. She's marked only on the boring, grade level stuff which she has to complete before she does her enriched math.

I tell her that grades are like currency - they're a tool to get what you want in life. Since her previous A in math got her into her enrichment program, she's motivated to get another A to get in again next year, so that means she's willing to slog through the grade level drudgery...