for a teacher of "regular habits"
enter the staff toilet and his/her preferred cubicle, drain cistern abd replace water with 4 litres of 20 vols Hydrogen Peroxide, drain u bend in toilet add 2 liters of 0.1M Silver nitrate followed by 2 litres of 2 M Sodium Hydroxide solution . this precipitates finely divided silver oxide..
when Toilet is flushed
4l of peroxide + silver oxide = 80 l of oxygen gas
Contents of toilet returned like Great geyser at Yellowstone usually to ceiling,, and stalagtites formed..
Warning, you Have to Really HATE the teacher...