Like Puncia said, Dont feed them. Act like what they say doesnt bother you.
I got picked on allot when i was in school as well, the staff did nothing, eventually, I just got bored of them saying the same things over and over, and everytime that they said summin to me, I either said something that agreed with what they said, Basically I laughed at myself and made fun of myself, it annoyed them so much, and when they still said harsh things about me, I just laughed at them and walked off.
Believe me from experiance, They hate know they cant bother you, it might be hard but just dont show them it bothers you, start as soon as you can and in about 3 days to a week, they get bored and move on, plus you get the satisfaction on watching Karma sneak up on the and kick their butts.
Believe me, A Karma victory is allot better then a Acting-Up Victory.