PBA, some kids did something similar to my husband when he was teaching in a chassidish yeshivah. Only in his case, what they actually did was remove the hinges from a class closet door, put the door back, and were waiting for him to open the door and have it fall on him. What a riot, right? Just hilarious!
What actually happened, however, was the Rosh Yeshivah came into the class for something, and opened the closet door to find it, and the door fell off on HIM! It was NOT a pretty sight, and the boys got into deep trouble. My husband was mortified, but he had no idea the boys had done this before he came into the classroom (apparently they were unsupervised for some time after limudei kodesh). What kids think of as harmless pranks, may not always be quite so harmless.