When I was at school, there were big analog clocks that sat high in every room, and they were all synchronised with eachother and to the bell system. So in year 12 we took all of the clocks out of every single room and hid them all in a few spare lockers in our common room. So when the bell went they had no idea what bell it was for. Fark it was hilarious! Then a week later at our Monday morning assembly when the principal calls all of the Year 12s up to congratulate them, we hid the clocks under our blazers and handed them over to the principal when he shook our hands in front of prep to year 12. It was deemed the best ever year 12 stunt, as opposed to the other funny as hell dangerous defacing sh*t we did So my advice is fark with the bell system, or pull a random fire drill without any of the teachers knowing lol