The SIMPLE explination of this is that School Officials/Administrators want policies that they don't want to have to make a judgement call on. The less you have to think about whether or not to punish someone then the easier it is for them. Also from a risk standpoint if you let this slide (in their minds) perhaps he brings the knife back to school and stabs everyone.

Either way all that was needed was someone to just have the balls to make a decision by themself instead of going by the book and being a damned bureaucrat.