Wow! you have gotten lots of great suggestions! I have an 8th grade DD 14 who hasn't seemed to experience much by way of bullying, but has surely seen it. My DD14 is especially sensitive to bullying as she lost a friend and team mate at the beginning of the school year to suicide due to bullying - all the schools in our area were already in a big "kindness" push and that really has amplified the schools efforts. Bullying in middle school seems to be an old, old problem though as my DS17 told me once that "even the bullies get bullied in middle school" when ask in response to a school survey.
Since your DD is athletic and enjoys her sport, is it one that she could perhaps "mentor" some younger kids or are there older girls in the sport(s) who would consider mentoring her? At this age / grade level - a lot of these kids need "service" hours for various groups they participate in so it could help her or an older girl to "do something." In our town, the animal shelter is a place middle school and high school kids can do service hours and if that is available to your DD (or something similar) it might prove to be something that could be of interest to other girls / boys her age and class and become a "pet" (pun intended) class / grade project and might be a way to re-form old bonds or create new bonds.
I also agree with the others who have said that some times it takes a while for these kids to settle into their "tribe." My DD's friends were all the same when she was in elementary school but in middle school several elementary schools moved in together so to speak, so her group of friends has evolved significantly. I try to be as "present" as they will allow me to be so they all know I care (and that I will call them out if they are misbehaving!)