Indeed, it was the knife that got him suspended. However, in his defense the claim is that the prohibition applies to students (no scissors in the class room or knives in the student lunchroom available to servers?). PC runamuck - I suspect most inner city kids have them available even if they don’t carry them to school.
Not that I entirely disagree, but if you plan to show your kids a knife and boxcutter better run it by the office first for this exact reason.