A similar thing happened at my school when I was 3rd or 4th form.During the lost three years between school and Army I had various jobs including working ins hospital. I was an admin bod in latterly records but in the beginning in the X-ray dept. One day there was much sniggering among the other staff as a senior regional advisor departed after a snap visit. Apparently this lady was not liked and her cup of coffee may have contained a dose of picolax. Don’t mess with middle aged NHS admin ladies!
The Upper Sixth just about to go on revision break before A level exams all played a trick or two. That year 6 of them hijacked the coffee urn being taken from the refractory (yes it was a grammar school) to the masters common room and added a very large quantity of laxatives.
They had the desired effect and very few masters around school for the rest of the day for some reason? The 6 were soon identified (the Lower Sixth duty chai wallahs who had been transporting the urn/hijacked soon succumbed to the threat of expulsion) and expelled, not even allowed to come back to take their exams which no doubt ruined a promising career or two.
The story made the media and one, Dave H, was quoted as saying "We thought the masters were full of sh*t so helped them get rid of it" or words to that effect.