Is this normal behaviour? Are these girls okay?

Okay, the girls at my school are VERY weird. They come up and talk to you, ask you for your e-mail etc.. That's all fine and dandy, but say this girl (who's nothing to you. Not your friend. Not your girlfriend) sees you talking to another girl (who might be your friend or even a cousin!), she ignores you or even hates you for it. I mean gives you real grief for it. Now magnify this by an entire population at a school. Is any of this normal? Are these girls normal? I mean, there are girl who you don't even know who'll hate you because you say hi to another girl or even smile at another girl. WTF? Should I just ignore ALL of these girls?