In my country they do the drills every year, the day - but not the time is announced in advanced. Everyone knows that they will be checking that everyone gets out, so we all leave and gather in our assigned locations and enjoy a paid break outside. They schedule these for good weather so getting outside is a nice break. This way people take it seriously, but don't panic. We know from experience that someone will be outside with a checklist to make sure we all leave.

There are scientific practices behind the above. I suggest you get your country on board with those so that people take the alarm seriously.

That's when you append "this is not a drill".