Why do girls give me a mean stare?

I know as a girl, I am mean looking. It’s one reason why people are shocked when they realized I am friendly. Lately I would be at school reading or studying and when I look up every once in a while to look at my surrounding, I would notice that some girls would stare and have that mean look on their face. I don’t know if it’s because of my face that look mean or it’s something else. Girls would constantly stare even if I’m not looking at them.