Kid Gets Bullied, Parents Sue School District

The parents of an Oklahoma City student are suing OKC Public Schools for "[failure] to appropriately address the bullying of their daughter". Apparently, their daughter was being bullied and harrassed by other students for two years and then was attacked on the school bus.

How exactly is the school the responsible party here? Is it because parents obviously consider a public school to be akin to a "free" daycare? I mean, really... A student or group of students "bullied" another student. That is a normal social dynamic of both kids and adults.

What happened to using bullying as an opportunity to develop a thick skin or to learn to stand up for yourself?