Those parents are hurtful to the child and school, and it's the child who is hurt in the long run. Also, teachers should not let themselves get involved when teasing occurs. It's very easy for bystanders to get vicarious gratification in watching others get teased. That's what slapstick comedy is all about. Teachers cannot let that happen.
Teachers have to make sure they communicate a value of respect among all the kids.
Whenever any teasing does happen, they must deal with it immediately. Schools need to really listen to kids who communicate a problem and if necessary, refer them for professional evaluation either within or outside the school. It's not a group phenomenon. San Diego has a tip number for drugs and violence, and I've heard it's been successful. Also, make sure kids know the importance of reporting.