Why do some teenage kids get picked on so much? Aren't schools suppose to have zero tolerance for bullies?

An ongoing problem. There will always be those who bully and those who are bullied. Unfortunately, the school teachers cannot see every activity. It is up to all of us (parents, teachers, doctors, other children) to help change the behavior. It will take time. Yes, zero tolerance may be a good start, but it will take a lot of work to get there.