In dictée, the importance of absolute silence from the very beginning to the end of the process, which lasts fifteen minutes at most, is not properly appreciated by most teachers, and that is a gross understatement.
On this point, Robert Harrell has written:
Usually I am able to impress the class sufficiently with the seriousness of the process that there is no problem. (I express clearly that I will not deviate from the reading of the text – no further repetition, no accommodation for “I didn’t get that!”, nothing but the prescribed reading of the text.) However, I have had classes that were unable to do this on the first dictation (Can anyone say “fifth period”?). I stopped, instructed everyone to throw away that paper and prepare another one. Then I went over the procedure again, and we started over with a different dictation. You generally have to do this only once.