“In” is a noun, preposition, adjective, or adverb.
Examples of “in”
I have an in with the company. (noun)
She placed her books in the bookbag. (preposition)
Dress shirts with no tie are in this year. (adjective)
Turn your essay in to the teacher. (adverb)
In the last example, “in” is connected to “turn”, “turn in”, indicating how. Put the adverb “in” next to “turn” and then it becomes clear. If one said, “into the teacher”, now there is a preposition and making the essay the teacher is impossible unless one owns the Good Witch’s wand.