Your child is written up at school, for example smoking, you say Not my Child! I say I saw him and wrote him up. The parents say to the student on the way out to the car, It's ok hon, want to get something to eat? Go to the Mall? OMG the child is rewarded for misconduct...Sound familiar?
When the parent gets a consequence do you think their child will continue with his/her behaviour? The parents are responsible for their child while in school. I suggest this be written in all school handbooks.
Call the teacher for a conference and include all of his/her teachers, also the principal. Let them know this is going on and make them aware of it. Let them know that your child has a right to a stress free education. If your child is being tormented in class and the teacher does not see it. The child should get up quietly and go to the office and sit. This will be a condition agreed upon by all parties. The other parents need to be brought into school for a meeting with everyone. This needs to stop. Remind them that the school does have a zero tolerance rule. If you don't get satisfaction let them know since they are very aware of the ongoing problem that if anything should happen to your child you will sue. Also use the media.