You have been asked to form a line, in twos. The teacher informs you that it is necessary to be quiet, so you must whisper. Upon this decree, everyone must (audibly) whisper the words "whisper whisper whisper whisper" to one another. The teacher will acknowledge this with a grateful "all right, thank you".

A substitute teacher at my primary school would often say, \'All I want to hear is the hum of your working.\'
It backfired on us though, as there\'s only so long an entire class of kids can maintain the drone of a beehive before getting tired, losing interest, or looking disapprovingly at those who kept it up for longer than the collective joke was deemed amusing, leading to them being singled out for beatings later.

A far more satisfying whispering prank. Firstly, locate a very old male teacher whoose failing hearing necessitates the use of one of those enormous spakka-special hearing aids on each ear. When the lesson commences everybody in the class must answer all questions in a whisper whilst at the same time maintaining the appearance of talking at a normal level. Repeat untill you see the teacher adjust both brick ear-muff hearing aids to maximum input volume. Wait untill the next question and then all shout the answer at the top of your voice (if you don't know the answer to the question, then a shrill high pitched scream will suffice). Now all sit back and laugh heartily at the sight of a man in his late 70s crying in pain due to the assault his ear drums have recieved from thirty spiteful 12 year olds.