I ran the school darkroom when I was in high school (yes, back in the day when we took pictures using film for the school yearbook). I tried several times to evacuate the school by claiming I had inadvertently mixed several hazardous chemicals. Never did succeed though, especially because the darkroom was a large closet in one of the science classrooms. The teacher there knew full well I did not have access to any of the deadly chemicals I claimed I had mixed :).
I did, however, excuse myself a couple of times from boring classes by claiming I left something or other on in the darkroom and that a fire would result if I did not run to turn it off. Somehow it took me a full 20 minutes or however much of class was left to turn off a simple switch on the enlarger :).
Then there was the teacher who threatened to seal my mouth with strapping tape for talking out of turn. I quickly produced a roll and dared him to do it (which would be at least a misdemeanor). He chickened out.