Make sure you don't slip into victim blaming when you talk about bully-proofing children and finding the right environment for children.
Sometimes kids get bullied because they happen to be near a bully.
Bullies come and go, in fact some children who are usually nice and kind-hearted can slip into bullying behaviour sometimes.
My lovely son, who had one of those "well-rounded, interesting lives with good parental support"- a friendly, independent, normally-good-looking-normally-sporty boy, who was always happy to try new things----was bullied in year 9-10, by a few boys who were being mean. They did things like get up and move if he went to sit down with them. They excluded and teased him and made him feel bad about himself.
In this situation, the only thing you can do as a parent is to be on the ball, to notice if your child seems a bit unhappy, and NOT to put moodiness and changed behaviour down to "hormones" or "normal teenage stroppiness".
Kids often are unable to tell parents about people who are bullying them, because being bullied is humiliating and because victims feel like the fact that they are being bullied means there is something wrong with them.
As a parent you have to be a bit of a detective if your child seems unhappy, and you have to immediately take strong action if the problem turns out to be bullying. I would say you have to talk to the school right away, and if the school is ineffective at dealing with the problem you have to change schools.
Most people (myself included) wait too long to deal with bullies, and try to get the victim to change something (often their emotional response to the bully or their actual response to the bully). I know that occasionally victims can turn the situation around, but most of the time the victim is not in a position to do that.