It rubs me the wrong way to exclude a girl from GS when she has yet to do anything wrong in that context. As others have mentioned, GS may be the thing that turns it around. I know it helped me quite a bit - I wasn't a mean girl, but I was pretty self centered and thoughtless, and some of the gentle smackdowns I got were very helpful in jarring me out of that and causing me to re-evaluate myself. Yes, even in 6th grade.
The first part of the year is a slower time for the scout troops I've been in. So why not use that time for a short session on the Girl Scout way of doing things? Go over the laws, the pledge, the slogan; talk about what they mean and how we live them in our daily lives. We always spent the first meeting coming up with rules for our troop activities; talk about being fair, being kind, being helpful. And talk about the consequences of not following the troop rules with both girls and parents. It may be that you or your co-leader have to monitor the new girl's behavior closely for a bit to ensure that she's fitting in, and be ready to step in with consequences immediately if she causes a problem, but don't write her off immediately.
Girl Scouts is supposed to be inclusive, and that sometimes means working a little harder to help a girl to fit in. I can remember a section in my old Cadette handbook for patrol leaders, talking about how to identify a member who was having a hard time and strategies to include her. I totally agree that Scouts should be a safe place, but that does not mean to make it safe by excluding someone based on what they 'might' do. If she definitely causes disruption and disharmony, that's the time for her to be removed from the troop for the specific behaviors. But she deserves a chance.
You would be surprised what young children take in from things that adults kind of brush past. When DS was 7 he joined Cub Scouts. One night he sprayed cologne in his eyes (don't ask
) On the way to the emergency room I was trying to get him to keep his hands away from his eyes, and I could hear him muttering, 'Akela, we will do our best!' over and over - he'd learned it in the first meeting the previous week. What I'm yrying to say is, your problem girl may hear the GS laws and really take them to heart, and have something new to base her behavior on.