He's talking in imperative tone but why "will" was used here?
Yes, he is talking in an official tone! The word "will" was used here to indicate something that is going to happen in the future. It is used both as an instruction and a prediction. In this case, he is stating that everyone in the room will rise when the judge comes in. Ex: Oh, sorry. I forgot your drink. Sally will go back and get it! Ex: Keep watching! You will see how the ballet dancers point their toes.
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