Why do girls/women exclude other females in groups?

First of all, I’m not looking for a pity party. I’m interested in your take on the subject because all I’ve heard from friends and family is, “They’re just jealous.”

I’m in a club at school where all the members are mostly popular girls. I wouldn’t say they’re particularly pretty or anything but they do their makeup well and always have nice hair and new outfits and they’re thin and I think the fact that they’re all upper middle class and outgoing has a lot to do with their popularity.

I joined the club at the recommendation of the teacher that runs it. I had a trial run with it last year and she liked the way I worked so she put me in the club.

At first, the girls didn’t speak to me at all. They just stared at me…literally. They just had dead, emotionless faces, and they stared any time the teacher asked me to do something or if I smiled at them…anything.

I’ve gotten to where a few of them will speak to me but they won’t hold a conversation. They’ll just ask me a question and that will be that.

Yesterday we took a group picture. They didn’t even invite me in to take the picture with them, I just happened to be standing there when they took it so I look like a goof standing in the back.

They posted it on Facebook and tagged every girl but me so I look even more stupid…like a loser that’s just in their picture.

I sent them friend requests on Facebook, only one added me.

I’ve tried to look friendly and be friendly, I try to act interested in their conversations and I sit in their groups and they just act like I’m not there at all.

It can’t be that I’m just “new” to their group because there are other girls in the club that are new that aren’t great friends with them that they talk to and hang out with now and have accepted to their clique, but me, they just pretend I’m not even there.

It makes me feel pretty bad and I’m just wondering what I’m doing wrong because I do like them, they’re funny sometimes and I like how outgoing they are, they just don’t like me.
And when any one of them speaks to me, it’s as if I’m an outsider. They almost speak to me as one would speak to a person who didn’t understand English. They talk kind of slow and look at me like a child.

Why is this?