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1. Turn your student book to the pronunciation guide on page 4.

2. Turn in your student book to the pronunciation guide on page 4.

Which of the above sentences is acceptable?
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Comments  (Page 2) 
I find both just fine.

If you all already have the same book open, then "turn to page 7" is fine.

If you want to tell them both which book and which page, then "turn in the red hymnal to hymn 332" is also fine. Perhaps the commas placed by MH are simply implied but there would be no pause in speaking this out loud.

Yes, "turn in" is a phrasal verb that means "hand over." If the sentence were only "Turn in your student reader" then I would expect the students to pass them to the teacher. But "turn in your [name of book] to page X" means simply "open [name of book] to page X."

You would NOT say "turn in to page 6" - you need to specificy the book for this to make any sense.
Teo1. Turn your student book to the pronunciation guide on page 4.

2. Turn in your student book to the pronunciation guide on page 4.

Which of the above sentences is acceptable?
I also think both sentences are OK. To lessen the possibility of misunderstanding turn as the phrasal verb turn in, you could change the word order of sentence 2:
2a. Turn to the pronunciation guide on page 4 in your student book.
Students: We have free audio pronunciation exercises.
Or, though maybe not as good,

In your student book, turn to the pronunciation guide on page 4.

In any case turn does not go with in. in goes with your student book.

CJ