The other kids need to be taught that being a bystander and not intervening is not acceptable.
"How can a parent make a difference? This article highlights ways you can support your child, not through asking him or her to change, but by identifying and supporting those children who are bystanders willing and able to act in your child's defense. Too often, as Coloroso notes, bystanders – those who are neither bullies, nor direct targets of bullies – are unwilling to come to the aid of victimized children because then they become the targets, and are sometimes treated more harshly than the original target. It may be because, by standing up for the rights of the targeted child, these defenders have called attention to the bully's inner character problems, thus adding insult to injury in the bully's calculations. Their outspoken behavior challenges the bully's right to harm others...
But even serious bullies are reluctant to target a child who has a strong circle of friends or supporters around him or her...every classroom and neighborhood has a good chance of containing other children who are morally motivated to protect and defend children who are being targeted. Unfortunately, far too often, teachers, parents, and others ignore and fail to support these children in right actions, because they tend not to be troublemakers, nor victims, and thus are a group of children teachers often feel they can safely trust with less attention. Yet identifying and supporting these children may be the surest solution to bullying, either of children with Asperger's, or of children targeted for other reasons."
http://www.learningdisabilitiesinfo.com/bullied-children.html