In answer to your questions:
1. Several over the years.
2. Nor one. No chance. Never gonna happen.
3. Most of them.
As several posters have alluded to, the open hostility of the educational establishment to Law Enforcement is a major barrier to fixing this problem. And this hostility is irrational, not based on any actual incidents or problems, therefore it's resistant to change. My jurisdiction eliminated the SRO position, at the insistence of the School District. They are very reluctant to even allow officers on campus in the event of emergencies or actual crimes. They prefer staff and students make such reports after school at a police facility, so that students of color and LGBTQ+ students are not traumatized by seeing a police officer. That's not a joke....thats their actual position. These folks are irrational and believe their woke positions shield them from violence. Unfortunately the news out of Uvalde plays into their belief that there's no use calling the cops in an emergency.