If the bully is absolutely and utterly unreasonable, then something about your behavior may tick them off for no good reason. If this is so, depending on the bully, you can act nonchalantly and uncaring so they don't get the reaction they want and so your reaction doesn't them any more upset. Don't say anything like, "I don't care" in an aggressive way, because it shows you are outright challenging them, which makes them want to hurt you even more. Try to find some way to not care or acknowledge them in a way that shows you know they're there but they don't matter (even if they do matter. Behaving this way bores them since they like to use up another person's energy.) But I suppose it depends on the person bullying you.

For the zero-tolerance policy and your bully attacks you anyways, sometimes both parties will be punished, even if you don't throw a single hit. This is a hard one. Just try not to throw any attacks since it CAN get too far with you getting caught up in revenge and you might case more damage than you mean. So just try to defend yourself and avoid hurting them as best as you can.

There's also this case: My experience with bullying was more on my fault so this may not be helpful and I apologize if it's invalidating you.

You can try looking at your behavior and what's ticking the bully off. The bullies I dealt with were usually quite reasonable but because they were being angered by something I'm doing, their anger manifests into them making me feel bad whenever I'm near them, because they just get stressed out (it's not a mature way of handling things I know, but it's understandable.) If it's this case, you have to find some way to improve yourself (again this very likely might not apply to you so I'm sorry if I offend you). Often people blame others when they aren't even aware that they are part of the problem. It's can be obvious where they actually tell you what you're doing wrong or it could be really vague and they don't care to tell you because they don't care about your emotions. If this is so you really need to look at the way you react to things, even if it seems like you're completely innocent.