It is your worst nightmare: your child is being bullied and the harassment is escalating. All the practical solutions (teach your child to walk away, don't engage the bully, use the "buddy system," etc) have failed. Your child is increasingly depressed and anxious. Therapy will help with the child's coping of this intimidating situation but the situation itself persists. The child whom you cherish continues to defend him/her but the wounds may be deep. Your child's focus should be on "normal" kid stuff - play, study, love and growing independence.
What to do?
Reframe this vexing situation into a positive module for life lessons. This may be your child's first experience with injustice, but it certainly won't be the last. There will be plenty of times when, like all of us, your child is treated unfairly - at school, at work, in relationships - and the skills you teach now will help your son/daughter through the next mistreatment.
Bullying: when parenting efforts fail to protect a child: legal recourse
First things first, stop the bullying child. Most parents would agree that it is reasonable to monitor children's web postings. If your child is being cyber bullied, then the bully's parents may be responsible for their failure to stop harmful posts. Offer to meet with the parents for a non-legal approach: mediate with a therapist, demand that the bully enter into therapy and have that student sign a contract to stay away from your child. Neither student will want this event known amongst their peers and the promise of quieting parental involvement may go far towards a successful outcome with the bully.
What about the school? If you have spoken to the teachers/counselors without success, head to the principal. (If you know that none of those avenues will work, bypass those and head straight to the superintendent). If the superintendent is reticent about meeting with you, just head to that office. Sit in front of the receptionist, politely, and wait. Make sure that the receptionist is aware of your unsuccessful efforts to secure an appointment & sit. Don't bring a laptop, iPad or book. Just sit there. Human nature being what it is, you will eventually make your point and will likely secure your appointment.
Once there, give the superintendent your diary & proof of efforts to resolve the bullying at lower levels. You should have a diary, the school handbook, other instances from other parents (which means your involvement in the PTA, of course) and a list of legal & grass roots remedies available.
You can post actual facts and opinions on all social media (be careful of defamation; broad generalized opinions can be problematic). These days, posting a website is not difficult nor costly; if your goal is to encourage other parents to join you in effecting change, this is often a most efficient avenue. Research class action lawsuits against the school district for its failure to protect the children.
There is a lot to do. A lot of research, planning and action. A lawyer can help with the plan for the cost of a consultation and there is always the potential for litigation. But filing suit should be the last resort, for it is expensive and may not have the same life planning advantages of a parent working directly with a wounded child.
Use life remedies first and involve your child. A pro-active offense will always win over a "sit back and take it defense!"