I know, I know. This is way hard. But believing your child couldn't be mean isn't unconditional love. It's denial. Really loving your son or daughter is accepting that good kids can do crappy things sometimes. Think about the last time you got an edgy e-mail. Weren't you tempted to fire off something equally cutting? Now imagine you're 14, with an adolescent's poor impulse control.
Add to the mix that today's technology makes it extremely easy for the line between target and perpetrator to blur. For example, Boy A breaks up with a girl and Boy B asks her out. So A goes on Facebook and spreads horrible rumors about B, and B retaliates in kind, believing his actions are justifiable self-defense. Now everybody's gone over the top. Or kids may decide, out of righteous indignation, that they have to punish someone, as was the case with some boys and girls at a school where I was working. They thought a boy had snitched to the coach that they had skipped practice, so they persecuted him relentlessly, at school and online. This may seem strange, but none of these kids thought that what they were doing was wrong. And be warned: Once a kid is certain he was protecting himself, he's often frighteningly good at convincing his parents that he's completely innocent. Before you decide you know who did what to whom, ask your child, "If the other person were right here telling me this story, what would he say?"