I too am a Junior GS leader and have been for 20+ years. I think how you handle this depends alot on the age level you have. We have Juniors and we simply do not tolerate that kind of behavior.
I know alot of parents these days do not hold their children to the standards we would like. That doesn't prevent me from holding them to a higher standard at scouts. I correct their grammar, their behavior and their attitudes all the time.
WHen parents drop them off for the meeting I remind the child to say goodbye, thanks for bringing me, and I love you all the time right in front of mom and dad.
These days too many parents are trying to be their friends and to be liked by their kids that they forget to be the adult and set the rules.
The first night of scouts we go over our rules. Some are housekeepping but some are also abouthow they are expected to treat each other. I know there is also this thing that happens after the CHiristmas holidays where the girls in 6th grade turn a corner and start acting like snotty teenagers. (I know that's a huge generalization but in my experience most of them do this) That can be frustrating but I have to remind myself they are testing me and my leadership and I have to be the role model they need and not allow them to grow up faster than they need to.
Some more practical strategies are to split them in groups and partners and I decide who they're with. My co-leader and I make them all get along. (my co-leader and I started at the same time so we've been doing this a looooong time)
We went to a play last Sat and I mixed them up when we seated because I knew the friends would talk. That's one way they learn to like someone new, when they have no choice but to work with them or sit with them.
Another suggestion might be to do a badge on etiquitte or diversity or community. SOmething where they learn everyone is different and that's a good thing and here is how we treat everyone respectfully.
If it is bullying going on you definately need to approach the parents of the bullying child because that is something that needs to be stopped as young as possible. Otherwise I think involving the parents who are already not holding their kids to a standard might not be effective. The girls need to not only know what will not be tolerated when you are in charge but the weaker ones need to learn from your example how to not tolerate it for themselves.
I hope this helps. The girls are very lucky to have you.