Best time: Having to take an essay or test and you aren’t prepared. A false alarm could lead up to the end of the period, therefore allowing you another day to study.
Worst time: Taking a dump in the bathroom especially in the rare case a horn is installed. Either you get out of there as fast as you can or you stay in the stall till the fire alarm stops (unless someone walks in to evacuate the restroom). I know someone in my school who had done that during the safety drill period and they didn’t get caught surprisingly.