Fight back. It can't be said enough, fight back. The stuff that's going on in school feels really important, and I don't want to belittle it, but it's ultimately meaningless whether A hates B or C knocks down D. What does have meaning, however, is your capacity for self respect, and if you don't fight back you'll lose respect for yourself.

Also pay attention to your surroundings and don't expose yourself to dangerous situations.

As to why there are bullies, you just have to take a step back in your perspective to see that it's an natural system present in all communal species to enforce norms where other corrective systems aren't present. Dolphins, dogs, lions, chimpanzees and essentially all animals that live in groups and improve their survival chances thereby have mechanisms to push behavior towards a norm, the bullies are just playing their role in the system. I suspect that if you examine your own behavior closely you'll find that you do some of the same things to people who are further away from center than you are.

Later in life the bullying behavior changes it's tone to be less direct and physical, but you'll always have to deal with it. Learning how is one of the few meaningful lessons you'll get out of school.

Good luck.