S.R.
With my dd, I find that a lot of "bullying" (if you want to call it that) happens under the radar. Girls say things out of earshot of adults...or it happens on the bus when there is little supervision.
I think all kids go through some of this and it's hard to draw the line between misbehavior and bullying. My dd did have a problem with one girl in 3rd grade and I specifically asked that this girl NOT be in the same class in 4th grade.
There were two other neighbor girls who were mean to my dd on the bus...so we quit riding the bus (it also gives us extra time).
My point is, the most effective way to stop it is to remove the child from the situation or notify an adult or teacher so they can keep a distance between the kids.