Active shooter drills aren't about preventing child gun homicides. They're about active shooter situations inside schools, which are actual events that have happened.

You may debate how useful they are, or if the frequency of school shootings is sufficient to justify them, but I will point out that the number of times I have had to evacuate a school (admittedly, due to a bomb threat) is equal to the number of times we had to "assume the position" for a tornado threat (exactly once).