And, to be fair if scores of mass shooter drills are held every week, a few of them are bound to coincide in time and space with random events.
To be paranoid, the shooters find out about a drill and schedule their shootings accordingly.
To be really, really, really paranoid, both events are scheduled by the same entity.
To rise above the random chance level real research and actual math is required.
But somebody intent on committing a mass murder might think doing it at one school while a planned drill is held at another would further ensure confusion and delay response, because everyone is preoccupied at the drill.
This may be an error because the 1st responders are all ready geared up so it may actually decrease response time.