A part of the answer might have to do with the developing brain of a teen. Specifically, adolescents are wired in ways that lend to risk taking when in the presence of their friends.
Researchers at Temple University tested the brains of adults and teens by attaching them to brain scan machines while simulating a driving game. What they found was that when teens were not observed by friends, they drove basically the same as the adults in the study. However, when teens had friends observing them, something interesting happened. A part of the brain associated with reward lit up and the teens took more risks. They had more crashes and reckless driving behavior.
This study lends credence to the idea of teens doing stupid things together that they would not do alone. The psychologist who ran the experiment, Laurence Steinberg, a national expert on adolescent development, thinks this finding may be applied to other areas of teen life like bullying. Maybe, the peer approval and possible social advancement associated with bullying is enough to light up the reward centers in the brains of teens who bully. He suggests that the short-term pleasure of the moment with peers may override judgement. This also means that giving teens more information on bullying or any other negative behavior is not going to prevent much. In these cases, knowledge is not power. It takes maturation to make good decisions.
Instead, it would be better to limit opportunities for immature judgement that could harm others. For example, I didn’t allow my teens to drive with other teens in the car when they first started driving. I limited the opportunity, knowing that the risk taking increases when other teens are present. In the case of bullying, working on the peer group to approach bullying as a negative and not a peer enhancing activity would change the context. When teens come together to advance kindness and empathy, we may have an effective strategy. And those values are usually taught at home.
Source: Steinberg, L. (2007). Risk-taking in adolescence: New perspectives from brain and behavioral science. Current Directions in Psychological Science, 16(2), pp. 55-59.