What would you do?!

So I've posted before about me working at a school with kid's ages 3-13. There is this young girl that likes me she is 13; she considers me a friend. I love all my kid's they are special to me and I make sure they feel that way. Well the girl came to school today and told me her grandma said I'm only nice to her cause she's black. It shocked me and I didn't know what to say to her; I just told her that it wasn't true!!

I was very saddened at how she said it and now I feel like anything I tell her she won't believe. I seen her grandma today she just smiled, shook my hand and left. I wanted to tell her so bad that I'm nice to her granddaughter not because of her color but because she is a intelligent, beautiful little girl and I see great potential in her but I didn't want to get her introuble. She even let her stay with me alittle longer while she went shopping.

Edit: it is her grandma not mom.