It depends on the school really. My school liked to project the image of having no real problems with bullying, and this was for the most part true... Because they rarely picked up on it. The problem doesn't exist if we can't see it, right?
With that said, as soon as they start to pick up on it, my school does get involved pretty quickly, though how successful the intervention goes is largely dependant on the teacher responsible for the situation. When some guys ramped up the homophobia against me about two years ago now, I went straight to the now Vice Principle and... Well, it ended pretty quickly, lets put it that way. Though it may also have been down to me pulling off an needlessly complicated stunt to expose them all as horrible, horrible people in front of the entire school. It was great fun.
However, my friends sister was being bullied for a long time by a number of different people and it went mostly unnoticed because it was mostly emotional. When the school finally caught wind to what was going on, the teacher who was involved was completely useless. Information wasn't passed along properly - her science teacher didn't have a clue what was going on and it led to a really dangerous brawl in the lesson; nothing was ultimately solved and the closest we got to a solution was the suggestion that she move to a different school. Really.
As for my primary school, they just sort of let it happen. I was bullied on and off by a group of really vicious girls for pretty much all of primary school without much more that a quick talk. Can't say much more about that, really.