Sadly, most zero-tolerance policies exist because getting sued is simply too expensive for the schools to deal with. Why make a nuanced judgement when you risk getting a lawsuit by trying to be fair?
No, zero-tolerance policies exist because they are low-effort ways of being seen as addressing issues of political concern (and because minimizing discretion of subordinate staff while avoiding creating an incentive to kick sensitive decisions up the chain is itself desirable to decision-makers); excessive restrictions, like insufficient ones, are sources of lawsuits, if schools were concerned about maximum mitigation of legal risk they would have more carefully tailored policies.
EDIT: It’s worth noting that zero-tolerance policies are sold as necessary for mitigating legal risk, but that's because that’s a more palatable sales pitch than “we want neither to permit subordinates to exercise judgement nor to have to consider details of individual cases ourselves”.
I guess I fell for the marketing. In any event I detest zero-tolerance policies for just about everything.
Take disciplinary action for example. Very few schools have the same rate of expulsion/suspension for students of all races. How do schools prove to a jury that this is not a result of discrimination or racism? Quite difficult unless they have a zero-tolerance policy.
(No idea if it would go anywhere in court, but when has that ever stopped someone?)
It all feels good for many people.
School policy has exactly 0 legal weight. It won't protect the school against a lawsuit. Maybe the low end of layers believe it, but there's a reason lawyer's compensation is bimodal.
But by the time school administrators figure this out, they are typically being offered an out-of-court settlement with a confidentiality clause.