The flaw is that the bully can pick their moment and thus avoid being observed. The victim doesn’t get to choose the moment and means by which they have to defend themselves. And the school will enforce zero tolerance only against what their staff witnessed, not the original bullying that is (from their perspective) only alleged.

The key rule about fighting back that is never taught is: don’t get caught.

Yea, you don't have to fight back in order to be punished under "zero tolerance." You just have to be involved, including as the victim. Kids get punished all the time for rolling up into a ball while the aggressor beats them.

"Justice is expensive and uncomfortable, let's just use collective punishment on everyone in the immediate area."