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.
It's called "definite generic" (as opposed to "definite unique") as discussed in "Definite generic vs. definite unique in L2 acquisition" by Neal Snape in the Journal of the European Second Language Association (JESLA).
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?
No. The substitution is not primarily a literary device. The sentences carry similar information but are expressed in different ways.
in the classroom today
The is an anaphoric that assumes that the speaker and listener are already aware of the particular classroom that is being discussed.
As it happens the poster, Blackknight, above explains this specific instance (so I don't have to):
I'm pretty sure that's just a form of synechdoche. The phrase "the classroom" is not substituting for "classrooms" though, but "schools" (which have classrooms as their parts).
It should not be assumed that all such the noun -> nouns are synecdoche.
in classrooms today
This is the plural of in a classroom today. which means in an example of a classroom. (The indefinite article implies "one [typical] example from many.")
Thus = in examples of classrooms.