I'm not going to invite a girl in my class to a party
Why don't you want to invite this specific girl to the party?
She wouldn't enjoy the party. She seems kind of lame to me.
It makes sense that not everyone is close enough to be a friend.  It is fair not to invite her as long as it is not being done in a way that is mean or purposely excludes her.   Being mean in the process can make you look like a bully.
I don't care what people think. Life isn't fair. You see she might have a mental problem so she doesn't get it. She also acts like she's better than me.
It seems like you are not inviting her to be spiteful. If this is the case, you should reconsider and instead use this opportunity to get to know her. It's wrong to exclude someone without extending an invite first.