Often myself I feel very bad and horrible and disgusting to see other kids getting picked on, or made fun of, or anything of that sort. I also feel desperately bad when I see kids eating lunch by themselves. I think its because I understand their positions. The chubby kid with acne who eats lunch on his own and pretends to not notice that he is alone (headphones/book) but you know he doesn't have any friends. That is me... on the inside.
I also got made fun of a lot in elementary school and somewhat in middle school/high school/still. Teachers observed this behavior. In elem. school it was very obviously not "kidding around" (as it is now), but no one did anything. Today though, in this society, teachers should be doing things! (Let's face it, the 90s and before did not have anti-bullying acts and the like).
Maybe you should voice your concern to a teacher? I would. I am fortunately very capable of talking to teachers about this sort of thing... so I would give it a try if I were you. I would say maybe "I observed so and so being picked on and I don't think it is appropriate or helps make this a learning environment for him. I just wanted to let you know." Don't make it sound like you're upset with the teacher because you know they saw and didn't do anything! 'Cas teachers are kind of oblivious sometimes... especially at the start/end of classes, when most bullying goes on.