There are two issues for discretion: (1) whether an offense occurred and (2) how much tolerance to use in punishing that offense (zero). Stage (1) is analogous to the trial phase and stage (2) is analogous to the sentencing phase.

The discretion is breaking down in (1) where school officials determined that these students violated a weapons rule, by the pretense that the student knew or could have known that their eating and food service utensils would be seen as weapons. So it's not an issue of zero tolerance (stage 2), but that students aren't receiving due process at the stage where they are deemed to be violent weapons offenders in the first place (stage 1).

The zero-tolerance polices are being applied in a way so as to make offenses out of normal behavior that any reasonable person would interpret as not being in the reach of the underlying rule. I.e. would a reasonable child understand in advance that a cake knife is a dangerous weapon?

So the zero tolerance policies are being applied so overbroadly that a reasonable student can't determine in advance what behavior is "violent" or what normal household objects are "weapons".

Overbreadth is a due process rights violation, a 4th Amendment claim, I believe. A group of activists constitutional lawyers would basically have to take on these cases, and start suing school districts for violating due process rights of students as the rules are being applied with such overbroad discretion that students and parents aren't being put on reasonable notice what behavior can get a student punished. Because of the zero-tolerance rules, (the second, sentencing stage) the consequences of due process violation in the first stage are dire.