That's pretty close to "almost never happens" That works out the 33 a year, but there's something like 129,000 schools in the US. And when people talk about school shootings, they're typically talking about mass shootings, and there are even fewer of those (apparently 0.5 per year according to https://publications.aap.org/pediatrics/article/153/4/e20230...).
And then there's the question of how a cell phone is actually supposed to help in such an emergency. If anything, I think it would be a liability.
As a parent that experienced such an event, my son having his phone let him message me that he and his class were literally running away into the woods and escaping the situation.
Obviously, knowing his class was ok was a huge relief, but also being able to talk to dad helped him calm down a bit.
And then there's the question of how a cell phone is actually supposed to help in such an emergency. If anything, I think it would be a liability.