My daughter and one of her friends were being bullied by a nasty piece of work, girl in their year 9 class, on the bus on their way home. They were getting slapped in the face, and on one occasion my daughter was punched in the face by this little bitch. When they complained about it, they were told by a teacher that they must have done something to warrant the bullies attention, and that because it didn't happen on school property, they couldn't do anything about it.
When I was told about this I went to the school principal and let him know that it was his responsibility to do something about it, and that kids had a right to feel safe at school, including on their way to and from there, on the bus. He gave me a similar story about not on school grounds etc, so I informed him in that case, our next stop was the police station to make a formal report of assault.
He soon changed his story, and the girl was given a weeks suspension (probably more of a reward for her anyway). I found out later that it would have been the second report, had we done it, and that she got off with a warning on the other report.
Of course, the biggest risk is that it may cause retaliation by the bully. But I think in this case the girl's parents must have given her a bit of grief about it. They didn't want their daughter having a police record at the age of 15.