I don't claim to know the answer for everyone in every situation, but there are a few points I'd like to mention:
1. It is illegal in Korea to beat students in school. Yes, it happens. It is still illegal.
2. I couldn't imagine watching a kid get beat until tears are streaming down their face without saying anything to anyone. I would feel I was complicit with the beating. I might not be able to intervene, but I can make it clear how inappropriate _I_ find their actions, even if it is just by telling a co-teacher. I've found that even if the person doing the hitting can't understand English, they know that you are talking about what just happened and that you think it is wrong. It might not make it stop, but it might make them think.
3. If corporal punishment was an effective means of behavior change, why do the same kids tend to be on the hall on their knees regularly? (There are studies that support the lack of efficacy - not just my opinion)
There are times I wouldn't intervene, because I have seen corporal punishment when it isn't being done maliciously or to an extreme. I still don't agree that it is appropriate or effective, and with one teacher at my school who uses physical punishment to an extreme, I'm planning to use Alex's photo intervention. And yes, it is a delicate situation, and no, I don't know how someone should draw the line, and yes, I realize that what is disgusting to me might not be disgusting to you. (sorry, this touches on a few issues at my school this week...)
Anyway, unrelated, Samuel, what was the point to the last line of your comment? Was it meant to be as passive-aggressive as it reads?