This sounds like a good time to discuss with your child that all people, even teachers and principals, sometimes make mistakes.
However if your child is the victim of the abuse mentioned, please do not stop trying to protect your child!!!!! If you have tried everything from both ends (i.e. you taught your child anti-bullying techniques, and you have tried to get the school to intervene and help the bully get appropriate intervention) and the abuse continues, even if to you it is “only” verbal abuse and mild physical abuse, this may be a reason to take your kid out of the school. This has nothing to do with the protectzia issue, but rather that as a parent you must make sure your child is in a safe environment. It makes no differences if the reason that your child is not safe is due to protectzia, due to ineffective school policies, or due to bad mazal of having an uncontrollable bully in the class, in any case you must protect your child.