I had the same experience at school, my ONLY friend in high school completely ditched me to go hang out with the popular kids... she left me to sit in the library on my own evey lunch time for a bout a week... then she decided that she wanted to hang out with me again once the 'popular kids' stopped inviting her to hang out with them.
Like an idiot I let her back in only for her to do it again and again and again every time she had the chance...
She knew I would let her back so she had no reason not to do it.
My advice... throw her away and find yourself some new friends.
Yeah sure it might be fun for a week or so but it will only happen again, she is obviously not a very good friend to you.