The bottom line is that all school bullying is the fault of the school...Period. There should NEVER be a moment when your kid is not within eyesight of an adult. No, you can't have teachers in the bathroom, but some form of personnel can and should be stationed at the door. Whether in the hallways, the cafeteria, the gym, or playground, the school is responsible for making sure eyes are on students at all times.
Now the school will tell you that they just don't have the manpower to cover every nook and cranny at all times...Bullshit! They are legally responsible for your child's physical and emotional safety before anything else. This means making sure they are always under supervision.
"But wait...shouldn't I teach my kid that they have to stand up for themselves and help won't always be around?"
To be honest I fully agree with this. But the problem is that the school system can't go on record and allow kids to fight, even in self-defense. It may seem unfair, but just imagine if they allowed this - It would be the Wild West. In this system, bullies have to be reported and left for the school to deal with.
As of late there have been strong anti-bullying campaigns and programs implemented in schools. These programs have been very effective in reducing bullying. However, do not count on the school to fully protect your child from being bullied.
"So what can I do?"
Tell you kid to seek out an adult, wherever they are.
Tell them to call you, or even 911 if they feel threatened.
Tell them to use their phones to record and document the bullying in real time.
Enter them in a good martial arts program - Believe it or not, martial arts actually teach how to avoid fighting at all costs, and only fight as an absolute last resort in self-defense. Martial arts also instill confidence, which makes you less sensitive to being antagonized. (See: Sports)
Cyberbullying can be just as damaging as actually getting "beat up"...if not even more. Today, kids have a strong media presence. Being shamed on social media can be devastating and has even contributed to suicide. (See: Social Media)
Summary
Like it or not, the strong pick on the weak. This is just biology. This natural instinct when combined with personality issues, can result in bullying. I know your kid has a phone...this would be a good time to use it.