I may I have said this one time before, but anyway:
I think it was in early 2007. I was bullied for a while by a rather chubby guy for being pretty much the only one in my class without a cellphone, while also claiming that my family was poor (which was not true, btw). He was kind of a "famous" guy in the class, but his friends never joined him in bullying me. After about four/five months or so I had enough and literally said, though obviously in my own language, "Let's have a fight about it, then."
The fight was over in less than five seconds. I grabbed his shirt and pushed him head-first into a metal door frame. He started crying and yelling uncontrollably while some others called for a teacher to check on him. And that was it. He never touched me again and I was one of the most respected guys in my class for the next two and a half years.
The teachers dealt with the situation very professionally. Both he and I went to several meetings to discuss what happened, and also had a few meetings long afterwards to make sure that nothing new had happened between us.
As for me seeing others being bullied; no, I didn't see a lot of that. But the few cases I saw I usually tried to stop, either directly or indirectly. There was this one guy who only had/have one hand and therefore had to take a decent amount of shit from others though. I regrettably didn't do a lot to stop that from happening, but as time went by he later ended up becoming one of my greatest friends, so I guess it kind of solved itself :)