If he allowed her to wear it, the headline would be 'Principal let girl wear offensive dress' and the NAACP would be suing the school district.
Which goes back to my first statement:
"to back down from your principles in the face of adversity, is nothing more than surrendering to the will of others. "
If the administrator is religious and was 'principled' in his personal beliefs no amount of law suits would make him back down.
Irregardless of school 'policy'.