There's prejudice, then there's discrimination. They both exist separately from each other. For example one can be prejudiced but not act on it, and someone can discriminate but not actually believe in what they're doing. I think the boy that was bullied is an example of discrimination more than prejudice: maybe one kid started things and others joined in to "belong" ie discriminating by bullying, but not necessarily being prejudiced or even knowing what they did would have such a big effect.