The likelihood increases that people will not be taking it seriously when it IS real. Also, real assailants are going to start using that "it's only a drill" line to keep their prey off guard and buy themselves time.
I could see something like this playing out...
The police to people at the school after real shooting spree: Why didn't someone call 911?
People in the school office: The masked men told us it was a drill and not to panic. We keep hearing about all these unannounced drills going on elsewhere, so we thought that's what was going on. We didn't realize till after the gunmen ran out and people were really dead that it wasn't a drill.
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