Unfortunately zero tolerance doesnt keep bullying from happening. Living in one of the best school districts in WA state, we had monthly bomb threat drills, along with various lock down drills & the regular fire drills. Also had 2 real-deal cops on every campus in the city, anytime there was a school function.
The year after i left high school, the police infiltrated the student body at the school i was attending & ended up busting a firearms/drug ring of younger kids - these officers blended in with the student body, did homework; not even the teachers knew who they were. They arrested 9 people in relation to that sting, 5 or 6 of which were in school, the rest were older. We also had a few real bomb threats, stabbings in the hallways.. Hell there was even a "war" going on at one point..
This newest incident today, is sad, as were all the rest. Very unfortunate. Even more unfortunate, is the complexity of the fixes needed for the situation society is faced with.