This is a very insightful and informative Op Ed by high school senior, student leader, and Philadelphia Student Union organizer, Amir Curry. He offers the backstory of Philadelphia's implementation of metal detectors and criminalization of youth policies that the Philadelphia school district and increasingly more districts refer to as "emergency preparedness."

"I’m a senior at Science Leadership Academy at Beeber. Our school relies on the voices of its students to build community. These values have guided me through my high school career. Growing and learning in a community that encourages students to be actively engaged is how our school thrives, and one of the primary reasons our leadership has exercised discretion in choosing not to implement metal detectors.

But the recent school board mandate subverts that individual school autonomy. It will implement an 'emergency preparedness' plan in all district high schools, mandating walk-through metal detectors."

https://www.philly.com/opinion/commentary/philadelphia-schools-metal-detectors-policing-20190404.html