as a dude who used to be quite small and sometimes bullied, i'd like to say that while it's important for teachers to step in when somebody's obviously being hurt (physically, emotionally, or otherwise), things from the students' perspectives are always going to be quite complicated...in other words, there is a very real threat of the student being ashamed for being "helped" by the teacher, as charlene pointed out.
of course, this does NOT mean that you have to just let it be and allow the social dynamic to work itself out. what i'd recommend, though--and i think this goes for ANY bullying situation--is that it's important to know how the students perceive and experience the bullying. get as much of a sense as you can of what the overall situation is. is someone being picked on by almost everybody, or do they have support from friends? do the other students side with the bully or do they have sympathy for the student being picked on and see the bully as...well, as a bully? most importantly, talk to the student who's being picked on as soon as you notice what's going on, but do it in private, away from his or her peers. ask them if it's really a problem, if they feel threatened, etc.
i say this because i know that my own situation could have looked like i needed "help" from a teacher's perspective, but i never felt like i needed a teacher to step in and intervene NOR did i want them to! i knew i had real friends and that most of the kids in my grade would have stood up against whatever big guy happened to want to make fun of me or push me in the hallway between classes. in my case, i would have been embarrassed if a teacher had stepped in and done something to this or that bully ;x probably would have made things worse for me.
BUT...like i said, the most important thing is to know what the student thinks about what's going on, once you notice it. i would have appreciated a teacher asking me, "hey doug, i noticed _____ likes to pick on you when he thinks i'm not watching. what's up? are you okay?" and i would have said "yeah, i'm fine. he's just a jerk. everybody else thinks so too." but you never know until you ask. if that same situation happened and my response had been to burst into tears and say that i felt like somebody was really going to hurt me, then that's a different story.
i wonder what other people think about this. or other stories/experiences with bullying in school. i used to think a lot about it when i was a kid (there was nothing else i could do about it) and it'd be interesting to hear what everyone has to say.
as a general rule, though, bullies suck