I did once get the better of a bully only by having him a good punch in his face when he went too far. Now this bully was hated over all and except for a few, nearly all my classmates were willing to fight him with me if I wasn't able to handle him alone.
What I also know about martial arts though is that some bullies actually deem it more of a challenge when you know the basics of martial arts. And if those bullies know martial arts themselves, as well, then we come to the same discussion as firearms. Will a gun protect you when everybody else has a gun as well? A bully is always a weakling, but that "weakling" does not always refer to physical strength, but rather to mental strength, and people who are mentally weak will sooner resort to violence, and then one good karate chop can lead to a good kung-fu kick and before you know it, it's war.
Of course, rule #1 taught in martial arts these days is that you are never allowed to use them outside the ring/mat/fill-in-what-is-applicable-here. And even when you only use them in self-defense (which can basically justify anything) you must be sure you are actually only using it for defense (for which the Eastern armless arts are initially meant) and not for the offense (which you easily get yourself into before you know it). Martial arts are not the solution to everything (something a grand master should also tell you). And when your defensive action leads to a bully coming back with a group making you fight 20-on-1, then where did your martial arts get you? (I've seen that happening A LOT when I was younger).
In the end, all a kid can do when being bullied is seek help of a responsible adult who takes them seriously. All martial arts can do is making a bad situation worse than it already is, I'm afraid.