supervise the back of the bus on the ride home from school. >> it was always the same kids that went to the back of the bus. every once in awhile, they'd make a snide remark. i told the bus driver i didn't want to go back there. >> oh, my god. you're so fat. >> out of earshot of the bus driver, the group of four rowdy boys start taunting karen. recording the whole thing on a camera phone. >> you're so fat. you take up the entire seat. >> when kids bully, what they're looking for is a vulnerable target and a big reaction from that target. that's what makes bullying fun. >> triple sag? >> triple sag. >> triple z. >> look at her [ bleep ] stomach. >> but karen keeps her composure for the time being. >> i was trying to ignore them. i really was. >> you're a troll. you're a troll. you troll. >> ugly. >> the bullies seem thrilled by