i had trouble with this also, my daughter is now 8. this may not work in a bigger school, but what the parents did at our school is set up a monthly parent/child meeting. we have only done it a few times now, but the idea is we all get together at a home or at the park, and discuss social issues.
a parent group is what we turned to, because this type of teasing can be almost invisible to teachers, who have so much else to take care of anyway.
our first visit began with the parents talking about their school experiences..good and bad, and what we wanted for our kids.
we had the kids sit with us and talked about what friendship meant...this was an excellent time to see which kids felt they had strong friendships and which didn't based on answers. we let the kids speak through their parents if they were uncomfortable, and have even considered writing items and putting them in a box to be pulled out and read anonomysly.
i thik that if there is significat parent concern, you can invite the class parents to a meeting and talk about it. being open honsest and gentle will set a good example for the kids.
often, the parent of the bully doesn't know their kid is hurting others' feelings, and would like to know, but this is the time to be very diplomatic, perhaps have the concepts of non violent communication on hand...