For instance, I might say, "Overcrowding is a major concern in the classroom today" rather than "Overcrowding is a major concern in classrooms today". Is that substitution a literary device? The closest I can get is metonymy or synechdoche, which have similar functions but still don't match this specific kind of substitution.
I wonder if substituting the article "the" in the second sentence isn't so much a literary device but a stylistic grammar choice. Omitting "the" before the plural noun (zero-article), "classrooms," denotes a generic class or kind, while "the classroom" is a sort of mix between a definite article and a proper article. In either sentence, though, the effect is slightly different: while both refer to the general, "the classroom" suggests an idea or ideal while "classrooms" suggests difference but ties them together by a shared characteristic (i.e. over-crowding). Nonetheless "the" is a super versatile article which can subtly change the effect and tone of any sentence.