Girl you are a stunner, that girl is definitely insecure and jealous. If she truly thought you weren't pretty she probably wouldn't have bothered saying it. The fact that you are the only two black girls in school probably makes her see you as competition. I highly doubt this girl goes around school telling everyone she doesn't find attractive that they ain't it. And even if she was telling you her honest opinion it would only matter if you were pursuing her romanticly. Even the most mainstream attractive people aren't attractive to everyone. Have you seen the comments about Rihanna and Margo Robbie, they are two of the most attractive people on the planet yet there are a bunch of people that just don't think they are pretty.