Special case treatment invites lawsuits by those who do not get special treatment. Lawsuits are bad (expensive) for public school districts already struggling to fund teaching programs. No special treatment, no lawsuits.
Zero tolerance doesn't eliminate special treatment, it actually increases it. If someone breaks a rule, and you know that if you notice it, they will be expelled, you turn a blind eye more often than if you know that more appropriate action will be taken.