As written, "by" or "among" both sound natural while "of" is incorrect. There is a subtle logical distinction, however, between "by" and "among" in this context.
Just as "between" connotes a relation between two things, "among" connotes a relation among many (3 or more) things.
So if your paper is about the differences between learning a first language and learning a second language, and is using the third-year students in Golden High School as a case study, and is discussing differences that most or all of those students exhibit, then "by" makes more sense, since "language acquisition" is being treated as a single, shared experience.
If, on the other hand, you are comparing and contrasting the students as individuals who exhibit idiosyncratic differences in the way they learn a first and second language, "among" makes more logical sense, since there might be as many experiences of "language acquisition" as there are students.
I'd note, finally, that this is an academic distinction that I wouldn't expect even most native speakers to pick up on.