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OK.
You can also say «Откройте страницу 21…».


«Вы должны сдать рефераты (or, maybe, сочинения) к пятнице».

I don’t know what an essay is like. In our Russian school we didn’t use the word...

I think, that is true in Russia too.


In my opinion,

Заткнись — totally unacceptable
закрой свой рот — rude (something between acceptable and unacceptable)
Закрой варежку! — totally...