One Out of Tune Note Can Sour The Whole Melody

All classes have their own unique dynamic.  Every teacher knows who works well with who, who is the class clown, who needs reminding to stay on task, and who is the "trouble maker".  Well what happens when that one "trouble maker," or student that knows how to throw off the balance of the classroom, leaves and a new one takes his/her place?  This is a dilemma I'm now faced with.

Last year I got to know the girls in one particular class very well.  I knew exactly who was the ring leader of the "popular" group.  Having one of these groups makes the class dynamic interesting.  To completely stereotype this type of group, they are the ones who try showing that they don't really care what's going on in class and that they are "too cool for school."  The ring leader, therefore, is the one who can either make or break the class.  This student and I had our differences but in the end I figured out what would work with her and I started to balance out the class.  This year she is not with us anymore, but a new student has entered the group.

With out giving much detail, this student has not completely assimilated into the class yet, which makes her stand out a little and she does not know how I run my class and how the class responds to that.  Her personality is strong which makes her hard not to notice, to deny and to keep on task.  With only knowing her for a week, I can already foresee issues.  During individual work she is often off task and those around her want to be off task with her.  Even when the others realize this is not a good decision and try to get back on task, she is so undeniable that they become frustrated and give in to her antics.  

This is where I'm in a predicament.  I know that this could be a good and challenging environment for her.  It has the potential to take her strong personality, with potentially good qualities, and make a good leader of her with great responsibility.  At the moment, though, it's hard to try to work with her because of her newness and the fact that I have established a dynamic with the rest of the group that she is just not fitting into.  

I guess I'm afraid at this point to bring out the authoritative side when the rest of the group does not need that.  I don't want to single her out, but maybe it's best to since she's already doing it for herself.  

Any suggestions on how to handle this student to help return the balance to the classroom?