A long time ago one of my best friends underwent bullying from his classmates. I would see it everyday. Day by day it got worse, and I was getting worried. When I asked my friend about this though he just said that it was nothing to worry about. Of course I understood that he didn't want to worry me, but I still felt like I had to do something. Because it was agonizing to see strangers throw rocks at him or call him bad names.
Since there was no clear sign of this getting better, I decided one day to stand up to his bullies. I tried to settle them down, and talk rationaly. In the end though we ended up fighting. 
I was happy that I stood up for my friend though.
And afterwards, when we discussed the problem with the adults at our school, it eventually went away.