Normally, teachers would access the kid based on his / her understanding of English & also basic skills ( prob reading some basic alphabets, simple words, numbers, shapes , colours ). They required to score at least 65% to pass the entrance test.
I agree with AB & I doubt is the skin colour as well. When my kids first arrived, they have minor difficulty understanding the strong accent. But that has nothing to do with the ability. They took the entrance test at newton too & was offered a place immediately.
If you think your kid is academically good, probably she's not used to be alone with a stranger. Unnecessary stress can hinder the kid from talking. Try appealing again & asked for a second chance or try other branches.
I know DC refuse entry to any kid who does not even try to open their mouth to speak English.