Anarchism is a broad term so I'm not going to generalize but many (if not most) anarchist schools of thought say we need a system that prevents rulers to emerge. It's not the absence of order but the absence of hierarchy.

By bullying the kid, they created a hierarchy and in a truly anarchist society, people would have stood up for the oppressed.

How do you explain to these kids that an absence of rulers doesn't mean they can go around bullying everybody?

You should have said "read theory" and leave as any good leftist would have done.