Did you tell him that, unlike him, you prefer to teach your daughter to stand up for herself and not be pressured into advancing relationships?
Honestly, I probably would have filed a complaint with the school board and wrote an editorial to the paper with the title "Assistant Principal calls my daughter racist because she turns down a date to the Prom" and then a scathing article in the local paper where you explain that your 14 year old daughter 1) isn't allowed to date, 2) that said daughter is half black and the product of an interracial marriage, and that 3) teenage girls have enough problems in highschool without authority figures trying to pressure them into dating individuals at the behest of said authority figures.