A child who has a gift for math... an interest in math and affinity for math... a love of math... would typically be asking for extra math at home, and would therefore typically have had substantial exposure to above-grade-level math.

Creating and maintaining a list of these outside-of-school pursuits may provide documentation helpful for advocacy in the school system.

Crowd-sourced advocacy tips, resources, and information are gathered in this roundup:
http://giftedissues.davidsongifted....y_Advocacy_as_a_Non_Newt.html#Post183916