A "rite of passage" is a challenge that everyone faces that you need to get past to get to the next stage. It's a rite of passage for a child to move away into their own home. Here it points out that bullying should not be something that everyone, or anyone, should have to face and get past.

A 'rite of passage' is generally a ceremonial 'passing', an event which celebrates a transition from one stage of life to another. In this context, it refers to how being bullied doesn't make you a more mature person, or how it's not a necessary thing to progress in life.