I don't really get teased that often. Smalls school, most everyone is on friendly enough terms, or mature enough not to go around teasing other people. Most people. Even so, I'm not one of those people who would be a prime target, as there really isn't much that people will find bothering me. When they don't get response that they want, they tend to move on and find more livelier prey.
I just hate bullies and people who tease others actually. It isn't when they pick on me, because I couldn't really care less about what they say or do about me. Not only that, but I could defend myself quite aptly, and use my wit to give a better response, should I want to. But what gets me is when they pick on people I care about, or pick on people about things that aren't really their fault. (i.e. autism, a face deformity) That is because their is nothing they can do about.
If I see people getting teased, I tend to step in a little bit. If not right away, I will almost always approach them later. I mean.. it's just wrong. If you don't help you are just as bad as the person do what they are doing. Sometimes it isn't what you say, it is what you don't say. Silence can be just as killer as words can.