It's not. I don't know anyone that's ever been through an active shooter drill. The closest I know of is the college my wife taught at had an optional hour long seminar for teachers.
[0] A distressingly large percentage of students will be near some kind of gunfire while at school, yes—a whole lot smaller percentage will experience the sort that these drills address. The more common variety that way more will hear or see are targeted (gang, grudge, whatever) shootings on or near school property, not the mass shootings these drills are designed to prepare them for.
[1] https://nces.ed.gov/pubs2017/2017122.pdf, quoted in https://medium.com/s/youthnow/the-response-to-school-shootin....
In 2014 I was working IT for a private high school which had an active shooter drill on a yearly basis. I was told they started it two years prior because a student came onto campus with a handgun.
Both in southern California.
"Nine out of ten public schools now drill students and teachers to respond to mass shootings, according to the National Center for Education Statistics." [regrettably via Vox: https://www.vox.com/policy-and-politics/2018/2/16/17016382/s... but still, seems legit]