I wish I was more popular...?

I don't want to be one of the really popular people, but in our form, there are two groups of girls. The popular people, and the less popular people. And then there are me and two other girls. The two groups are very tight knit, and me and the other girls don't get on particularly well. So I'm always left on my own in class when we have to get in to groups. And I wish I was more popular, or had some sort of group to fall in to. But I don't, and it makes me really upset. I do things out of school, where I'm relatively popular. But I'm spectacularly unpopular in school, mostly because I have a speech impediment. There are no speech therapists nearby, and even if I did get speech therapy, I'd rather not be friends with people who are only friends with me when I change myself. What can I do?