I tell my kids to:
Move away from the bully,
If they follow them, shout as loud as they can at the other kid to leave them alone,
As soon as possible tell a teacher,
Tell me as soon as possible if anyone says or does anything at all,
If it happens again and the teacher doesn't do anything, then I go and talk to the teacher, and I make sure my kid sees me talking to the teacher, so they know I have their back,
6. If it still continues, and it is physical, I've told them that they can hit back, but only once we've talked to the teacher and the school knows that there is a concern, (it hasn't got this far yet),
7. If after talking to the teacher and it hasn't been resolved, I'll raise it to the headteacher,
8. Then I'll raise it with the PTA, and basically shame the school into acting,
9. If nothing happens, I'll raise it directly with the other kids parents,
10. Finally, I'll involve the police.
We've taken it to step 5 on my bully-o-meter, it has stopped now, not that I really think it was bullying, I think my boy was being a bit too sensitive, but I still made sure he knew that I was going to go to bat for him, and I spoke to his teacher with him. I've also gone directly to step 9 in playgrounds, when parents are not watching their kids and one of their children has hit my daughter. I am average height, but quite wide and I can be a loud mouth when needed. The parent could barely look at me. My daughter's tears stopped quickly when she knew I would protect her.
I remember being bullied as a kid (and also bullying unfortunately), and it went on for a long time. I told my parents, who basically told me to suck it up. They did once discuss moving me to a different school, which I really really wanted, but it didn't happen. I actually felt quite let down that it didn't seem to be taken seriously enough. I would skip out of school as I got older and missed a lot of lessons.
So, my advice is for any parent to be as active as possible. Make sure the child knows you are working on their behalf. I think my parents might have spoken the teachers and governors at the school (my Dad was a governor for a while), but I am not sure, so in my young mind I felt that I wasn't being supported. Obviously you can't be with the kids all the time, and if the bullying is happening outside of school, then it needs to be taken up with the parents directly. Confrontation is difficult, but as parents we have to put aside our concerns or inhibitions to help our children. I do understand that by confronting the parents it can make the bullying worse, especially with teenagers, but at least you've attempted some mediation. If it fails, then the police should be involved.