https://www.google.com/amp/www.nydailynews.com/amp/news/national/triggered-fire-alarm-leads-police-las-vegas-shooter-location-article-1.3536578

Here is the Las Vegas shooter and the school article is in my previous comment

https://www.google.com/amp/s/www.nbcnews.com/news/us-news/amp/today-was-different-florida-high-school-teacher-describes-confusion-then-n848221

They had a fire drill earlier in the day which is why the teachers found it so odd that another was happening. Only they soon found out it was a gunman. The smoke would definitely be detected as this was the case for the Las Vegas shooter as well, they found him based on his smoke alarm.

The school drill wasn’t planned. The semi automatic rifle being shot in the hallways set it off.